I bought one of the dual-diaphragm boosters from a user here. You'll probably have to wait a while and scour the forums and FB groups to see if you can find a listing or someone making a comment about it - that's how I found mine. I actually got it for pretty cheap too, because he had another one and that one he sold me was a spare, so he had no use for it.
I have been wondering about how hard it would be to try a B/E body Bendix booster on a C-Body - I do know the measurements for the pushrod are different, but how much that makes a difference, I don't really know. Seems no one has really tried it.