I just picked up several pieces off of ebay, so you can check there. Several online fish stores sell some driftwood as well.
My preference so far has been ebay, because you can look and see exactly what you're getting, as with most (not all) online vendors will give a generic picture based on size, not necessarily shaped the way the one you receive will be.
Make sure you read the article on preparing driftwood before you put it in a tank!