Hi John,
Great thinking to have a spare transmission around and ready to go in case there is ever a need.... right?
Seriously I hope you're not having too serious of issues with the buggy....
While I don't have a spare trans-axle laying around, you may want to try your local Craigslist.org, although a bit of a pshaw shoot and you may already have.
I can't recall your specific location, but I found this in the Austin area.
VW Volkswagen Beetle Bug IRS Transmission 1969-1979 - $50 (Round Rock) There's also
TheSamba.com. Again maybe you're already looking here too. I'm not seeing much there, in Texas at least.
Good luck and I hope someone else here can give you a better response.