Naturally occurring decorations/plants for aquariums?

I love the look of a completely natural looking tank. But LFS sell plants and driftwood and decorations for so expensive! (I'm not being cheap here, I'm just looking into something different if it's even possible.)

I know some people find driftwood and rocks to use in their tanks. As long as you clean them up really well! But what else can you find in nature that is safe for fishtanks? What about plants? Are their any plants you could find growing in a stream or somewhere that would work and be safe for a fish tank??