I mostly just go out and poke around shorelines and such.
I've had pretty good luck using Google Maps (satellite view) to locate potential sites - If there is a large accumulation of driftwood, it will show up on the higher resolution images. And then there will be plenty more when you show up in person.
Most of what I see looks to be from dimensional lumber, pilings, and bamboo, but I see stuff from regular trees as well.
I think it's just a matter of getting out there and looking, and having luck and stumbling across a nice rootball or branch or something.