I wouldn't use sprites to make shrubbery and plants, unless it's for really small things like weeds and grassy patches. They look really horrible for large objects, and we don't live in the Doom era any more, so it's a bit strange to see them. Instead, I'd use models for plants, which are actually quite easy to find, unlike sprites.
So, I'd tell you to check out
DoenerKing for a good source of models, but it seems that website is under construction or something. Since there's not too many other options at the moment, I'd recommend you check out Day of Defeat's GCF, as it has a ton of plants in it.