Pinterest is definitely the best here!
I like to think in collages/Pinterest desks/folders. First I make a separate folder for each character (or a country they represent/or their usual habitat/or just their wardrobe). I save everything I see fit in this folder. Then I look at it and find the elements that repeat a lot and start thinking how the clothes suit the character, the climate they are in, the situation they are in, how their wardrobe should change under different circumstances...
I also save a lot of little things I find interesting, that may be helpful in other designs.
Like I know my girl Angie here is from Vakhar, a country with pretty warm climate, with a lot if things going on on a seaside. Indian and Arabic aesthetic seems perfect.
So, it's colourful and has a lot of fountains and ponds. I also like a lot of greenery, so slap it all over the city.
If it's a hot country, they probably have clothes that are pretty loose. Most of them still wear traditional clothes, and have 2 main festivals, one is about winter and sea, the other is about summer and bonfires, so the clothes should look different too. So let's make them colour coded, they have different meanings for different colours, you can mostly see it in the children's clothing, because children are more vulnerable, and parents want them to be strong (black), healthy (red), wealthy (yellow). Also the country is pretty big, so the climate is somewhat different in different parts, so maybe add more pants to the wardrobes of people in the North.
But they always wear these big fabric belts, so, oh! these can not only be belts, but also shawls, or even skirts, so it would really be one piece of clothing used in different ways and it would be a tribute to their goddess, who created the sky from the same piece of fabric she wore. And the goddess also made an ocean from her eye, so they have eye jewelry as a charm.
So the girl Angie here has this or that element of her traditional clothing, but then she goes to the different country with a different climate, and her wardrobe changes too. Also she never wears red, so when she wears red, it's in a very stressful situation for her.
Overall, it comes down to Pinterest desks and one big travel in your character's clothing in your mind