Whitney Gardens in Brinnon, WA usually lists it. You might also check for rhododendron impeditum. Most rhododendronophiles believe much of what is sold as impeditum is actually fastigiatum. Should you actually get the genuine impeditum, it would make an excellent candidate for bonsai as well.

Rhododendron Species Botanical Garden Fall Plant Sale has both R. impeditum and R. fastigiatum. Don't have to be a member to order and these both are the 'real thing'.

Rhododendron Species Botanical Garden replied :) . Whitney's did not :( .