So i try to hoard as many places for references as possible but my start points for searching have varied from google searches to social media sites (i.e. twitter, tumblr or pinterest) and then just known resource websites. So all that said these are some places I've used or found from other folks
Poses
Design Doll by Terawell - program has to be downloaded on a pc/laptop
Magic Poser - link leads to webapp version but it also has a mobile app
AdorkaStock on DeviantArt
Kibbitzer on Deviantart (also has a Pinterest account with collected references they've made)
and for convenience sake a whole thread from here on the forums that compiles some more pose resources
https://forums.tapas.io/t/what-references-do-you-use-for-poses/70648
there's also the built in 3d models that Clip Studio Paint offers if you happen to use their software
Rooms, backgrounds and other environments can get a little more complicated depending on what you want to achieve but you'll likely be combining assets or elements from a number of different places. Again I'll note that there's folks who've built assets using CSP's 3d models and assets but there's also
3D Software
https://en.acon3d.com/
Room Builders
Map making tools if you want to get that detailed about setting
These tutorials from Vampbyte
You can also try photo bashing with images from sites like these
https://www.pexels.com/
Even Google Maps and Street View can be useful if you know how to fudge it
Overall any combination of these can be put to use along with just exploring images on a search engine or taking photos of your own. There's also just building or buying pose able dolls, play sets and dioramas to work from and build on in your drawings/comics it's just a matter of knowing what's accessible to you