I ended up cannibalizing an existing pair of Grado SR-60s for their drivers. If I could do it all over again I think I would have gone for the drivers from a pair of Fostex T50RP instead. They have a reputation for being good for mod projects.
It was nice to get a proof of concept built, but I still wish I could have found a decent supplier for bulk ordering drivers.
I had aspired to kickstart my headphone idea, but without consistent access to bulk components this remains a fantasy.
It's been suggested to me if I was prepared to purchase in bulk, sometimes large batches of various electronic components are sold on http://www.alibaba.com/. Trouble with this approach is I cannot order a sample, I have to buy a large quantity site unseen.
Lemme know if you discover a better solution.