I am not a real expert, but I sure do take an interest in these things. Always learning more.
One thing I have not learned is where to buy these darn things. Demand is up and supply is down. Prices are way up, and quality is not, usually.
I have most of a 930 box available, but it is in pieces. I lack the middle and end covers, and bellhousing side cover, and some reverse gear hardware to complete it.
I watch eBay, Rennlist, and Pelican all the time. A lot of the stuff you get is not as good as it looks. For example I bought a diff that someone said "looked perfect", and he was right, but it was 100.0% worn out. He took it back, and then 6 months later he still had it, so I bought it again, this time at a price that factored in the required repair (new clutch disc pack).
I do not recommend buying online from people you don't know, that aren't in the business, that aren't sure of what they have, and that don't have a reputation to look after, unless you are prepared to go through some trial & error and some iterations. That could include installing a transmission, then removing it, rebuilding it, and reinstalling it.
David,
ebay is always a good place to check. Even the 930 four speed is thin on the ground these days, there hasn't been a 930 come up on ebay for months now. I think G50/50's are even more unobtainable. Possibly keep an eye on Rennlist and Pelican Parts forums.
One high profile dismantler to be very careful with is GAD. See this thread
http://www.gt40s.com/forum/ebay-marketplace/21011-87-89-g50-ebay.html?highlight=GAD
Joel (jmracecar) on this forum is the G50 expert.
Good luck with your search.
Cheers