Reddit reviews Styrene Sheets, White, .040 x 7.6" x 11

Going to a pet store or model train store will get you a lot of cheap terrain that looks reasonably good. If you have some coin to throw around, I may recommend a nicer tree set, like this one

https://www.amazon.com/gp/product/B000JXW7OK/ref=oh_aui_detailpage_o04_s00?ie=UTF8&psc=1

I've bought a couple packs of those before, and they look frankly fantastic.

My forest boards usually have a mix of nice/expensive trees and cheaper trees to fill up the board. Remember to get some line-of-sight blocking hills and cliffs to, and it's always nice to have a building or two.

For basing I'd recommend styrene sheets. They're cheap, easy to cut (I just use common scissors) and work well with regular model superglue.