Roughly half of my ideas come from daydreaming moments; for example, I look at something and my mind just invents a story around it. A quarter comes from throwing around ideas/bs with friends, and the rest comes from (this is odd) dreams. I actually keep a notebook next to my bed, so I can write down ideas quickly, before I forget the dream.
Some come from songs, some come from everyday life, some come from other works in the media, but mostly.....when im in the shower 
I just build on old ideas I've had, or sometimes ideas come to me in the shower. (don't ask me why, because I don't know)
a lot of my ideas come when I'm just starting to fall asleep. It's incredibly inconvenient but I keep a notebook next to my bed....and well pretty much with me at all times
To answer the question though, everywhere sometimes I'll just see something and think "what if it was like this?"
Admittedly I don't have any work on tapas yet but I design from the top down starting with the core themes and points. Something I'm trying to induce in the audience based on what I think people will like. Or just what I want to see because I haven't yet, or haven't for awhile. Usually starts out pretty amorphous at first with just a logline and then gets more specific as I fill it all in. Most of the spontaneous ideas happen when it starts looking like an actual novel and not just bullet points. Things that'll fit or tie certain things together well or would "make sense" for the world I've created, etc.
I feel like I just get ideas in my head because I daydream so much. I've been publishing "Vows of the Sentinel" on Tapas and it's a story I've been writing for about 4 years. At first I just had the idea of the start of the plot line, and then I sort of figured out how their story might end, and in between ideas just come along as I write. Sometimes you just have to take an idea and begin working on it. It will develop as you go. I've literally had chapters that I'd planned out accordingly, and ended up with new chapters filled in because my own writing gave me new ideas. I've also had to research a lot for my story so that what I'm dealing with is accurate (I've got military themes and no military experience. But I've learned a lot about the army LOL).
I've also found it easy to work with dreams. If you ever have a dream where you think "That was really interesting, I wonder how it would have ended." you can start running with things like that. But usually my ideas start and are very unfinished until I begin writing, and my imagination begins to take over.
Or even conversations or things that happen in life. I've added things I've experienced to my own stories.
(I do reach writers block occasionally, but sometimes taking a few days away allows for you to regroup and feel ready to dive back in again).
And as mentioned above, I get influenced by music. I literally have songs that I feel relate to my story and lyrics within those have brought out new thoughts.
I base off a mixture of pre-existing tropes and work from there. I may follow a formula I already enjoy or throw in some unexpected twists, but I always start by combining 2-3 tropes such as "magical girls, smart-dumb pair, GL" and then build a story.
I know of one manga artist who actually plays a game where she flips through a dictionary and lands on 3 words, then must make a short one-shot based on those words!
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I do love it when inspiration suddenly hits like this! Sometimes I find a small trigger like a well worded sentence or a cool sounding name brings up loads of awesome imagery in my mind and from there my mind is just going mad with the possibilities!
That's cool, I really do think sometimes the best ideas are ones that 'I'd like to read' or something i'm yet to see! I guess it's true when people say the best ideas are ones that you wish you'd came up with!
I get my ideas from my nightly dreams and nightmares. I like to put them to good use instead of scaring the crap out of me.
I seem to get most of my ideas from listening to podcasts/music, or watching Youtube videos, or in real-life conversations with people. Just random tidbits a person says or does that inspires little things, even if the topic in question has nothing whatsoever to do with my current story (but can certainly flesh out characters and concepts all the more). I get more visual inspiration from shows, movies, other comics and art and stuff. Naturally 
I like reading/watching anime with unusual plot set ups as they tend to inspire me the most. They usually get my brain thinking “but what if THIS happened instead of THAT.” And I like getting myself into that brainstorming zone >:3
I listen to a ton of different music, and most of it is repetitive for a few weeks until i forget about it.
Why is music relevant to this conversation?
Because my #1 source of characters, scenes and novels themselves comes from the music I hear on the radio or listen for about a week.
My Writer's Camp entry is a novel that branched from the title and song "Natural" by Imagine Dragons. It morphed from an idea about humans in an end-of-the-world situation to a group of robots years after the end.
The characters of the novel were (In story order) inspired by songs such as:
Jonas Blue - Rise (though the character morphed to not suit it in development)
ZAYN, Sia - Dusk til Dawn
Selena Gomez - Back to you (I actually heard all these songs up to now back to back on radio and didn't know this one until i found it on youtube)
Lady Gaga - Bad Romance
As for my other novel, that needs some renovation,, I actually don't remember its inspiration.
Some other of my works have been inspired by mythology (an abandoned BL plot, my current work im going to be likely publishing) or daydreams (an old GL I had on Tapas, Liberosis. Now deleted.).
Honestly, they just come to me when I least expect it or when I feel the world of webcomics is missing something. When I get an idea and see everyone has yet to do it, I tell myself "I'm not going to wait for someone to do it. I'm going to be the one to do it". I take pride in this unique approach.
As weird as it may sound but most of my inspiration came from my dreams whenever i went to bed. At first started as something i find weird that was happening during my days in high school then suddenly they kept on coming and coming to the point i found it annoy but then i decided, "why not write it down?" and so i did. and from what i got turned in my hobby of writing stories. Although i didn't immediately published it because whooo boy did it need a lot of tuning before i finally published it. Other inspiration came from listen to music or watching shows. Sometimes from books from classes ....-_- even though i really hate some classes.
I just try to think creay tiv lee.
For realzies though I like to try and start off with something thats really really dumb and then fix it up from there until I think it's good, it's always easier to edit something til its good than it is to come up with a great idea.
For example, an initial idea might be something really stupid like "what if flies could talk?" and then I'll mull it over in my head for a good while until it develops into something less dumb. Perhaps in its final version it's a psychological horror story about a guy who suddenly can hear what flies are saying? Or maybe it's a goofy animal slice of life with a fly-person as a protagonist? Maybe I'll do some research on flies to find out an interesting fun fact that could spin off into it's own set of concepts (They only eat liquids! Maybe the fly guy is on a juice cleanse, or they're an alcoholic bartender?)
Eventually after mulling it over long enough I'll have something that I might wanna actually put in the hours/weeks/years to turn it into a real thing, and during that process of making this initial idea into a comic I'll probably come up with even more ideas for it, and for other future projects.
So many people shut themselves down before they even give their ideas a chance to develop, because they think they're dumb or that they can't make a thing about XYZ. The truth is is that no idea is good or bad inherently, it just depends on the execution of it. If you make a comic about an alcoholic fly man it will look really really different from the version that I make. That's just how creativity works.
