https://www.deviantart.com/ is a good place to find free pics of models (real) and I've ran across quite a few PNG files of props as well, and even backgrounds. There are groups for Photo Manipulation, and a lot of DAZ users as well (I'm sure their renders aren't for free however, copyright, etc.) There may be a few however if you just run a search for what you are looking for.
Aside from that, creating the 3D renders with characters is what DAZ is all about, and you can make your own in the program. It's not really hard to learn, even if you only use 2D programs, Daz studio has enough tutorials available to walk you through any questions you may have. And since you are a member here... the files are practically free... 50 points per most files. It would be the best way for you to get the specific files you need for your 2D pictures.
Good luck whatever you do..
Niniane