Whatever you can get to sink that wont harm your plants or fish really.
Rocks you have to worry about having toxins or increasing hardness, with the wood its mainly just the toxins.
Anything at an LFS that suits your fancy, otherwise I think there are some people on here that sell manzinita driftwood...but you would have to find one out your way.
I dont know what sort of substrate you have, but it looks like I might change that along with the decoration and adding a background if it was me. Plain colored backgrouds are better in the long-run if you are serious about the planted tank business I find.