A couple of resources I use quite a bit:
https://www.sketchuptextureclub.com - free textures! They have a "club" membership that gets you higher-res textures and some textures with more bells and whistles, but for OOTP purposes the free tier is perfectly fine.
https://www.textures.com - you get 15 free "credits" a day you can use. Textures cost more credits the larger they are, but the most basic ones only cost 1 credit apiece. They have a much larger selection than SketchUp Texture Club.