You pretty much need a bogwood. Anything else is going to break down and dissolve in your tank over time. Mangrove and cypress are examples of a good bogwood. Mopani from Africa is what most lfs
's sell, you just have to soak it for a few days/weeks to get all the tannins out. Most of your normal trees like oak or hickorywill soak up water, causing the fibers to swell up and split. When that happens the wood starts coming apart. Not good in an aquarium.
Search around creeks and streams and you may find some good pieces. just do as others have suggested and boil it, soak it in bleach and whatever else you have to do to kill any 'critters' living inside it.