Also look into similar supplies used to build model train sets and dioramas... They could be less expensive, depending, and are especially great for wilderness terrain and features.
A store like Hobby Lobby is a good place to start.
"S-Scale" (1:64) railroad models would correspond to roughly 1" = 5' (1:60).
Good point - I bought a bunch of trees several years back from a model railroad outfit... and, since trees come in all sizes (small cherry trees to gigantic redwoods & sequoias), you can pretty much get a range of sizes. From 1-2 inches tall to 9-12 inches tall and anywhere in between. Unless you want to mount them on your own, though, make sure you get the ones with bases on them.