I also found driftwood very hard to get through "normal" sources, so I
checked with the local folks who use it so much. It seems that, at least
here in central Iowa, driftwood for tanks is a Do-It-Yourself proposition.
Each summer, folks take hiking boots, insect repellant, drinking water, and
old clothes, along with a small, sharp saw, and walk the upper reaches of the
many artificial lakes in Iowa. They just saw off the really interesting
combinations of dead roots, branches, etc., that have been carried downstream
and left to dry out for you along the shores of the lakes. They "cure" the
stuff by sinking it into a water-filled plastic Stock Watering Tank
(Rubbermaid makes several nice ones at very reasonable prices) and let it
soak. Occasional water changes are also done. Sometimes really cruddy pieces
need a bleaching and scrubbing, too. Then they use stainless steel screws
and chunks of slate to attach bottom plates to the specimens.
It is kind of cold out right now to do this (except for those lucky folks on
the other side of the Equator, where it is early summer) , but you could try
it next summer.
Just a thought.