I just ordered one from Doug Green up on Vancouver Island.  He's the North
American counterpart to Groschi over in Germany.  Delivery is 6-8 weeks,
and its a little on the pricey side, but I need to replace the tank
anyway.  Its $550 for the tank, plus delivery, which isn't cheap because
of the size, and I'm figuring another $200 or so to have it installed.
Also, you'll want to replace the fuel sender, fuel lines to the overflow
tanks and the manual transmission bushing while doing the replacement.
Here's the contact info:

http://www.southafrica-parts.com/
doug.green@community.royalroads.ca
Telephone: (250) 655-1504

BTW, I have no financial interest in doug's enterprise.

Cheers,
Mike
'91 GL Westfakia
The Love Wagon