I buy pieces of driftwood from pet stores. In the past I used a piece of wood from my yard (I live in the woods) that we had sanitized in the oven, but I just feel safer using wood that I know was specifically produced to be used with reptiles. I've never had my wood grow mold or anything like that... If you're worried about something like that, some pet stores also have fake plastic pieces of wood (if you can't find any near the reptile supplies, check by the decorations for fish.)