I think PlantedRich addressed this pretty well.
It's no so important that it's actually driftwood (which is kinda funny, since very little of the commercially available 'driftwood' ever actually drifted - malaysian bogwood from stumps/roots, manzanita, mopane chunks, sandblasted grapevine, etc.), as much as almost anything that isn't lignin or cellulose has washed out/leached out/decomposed.
I'm a big fan of trying to collect it yourself, pretty much like PlantedRich stated. One thing I've found useful is to use Google maps to locate potential sites. This probably works much better with large open bodies of water (oceans, lakes, ponds, rivers) then with creeks and canals, but it would be worth trying.
And even if you don't find any suitable pieces of wood, there is usually plenty of other interesting stuff to see.