If you can't get a 2nd hand gearbox, then you can always get it rebuilt at Abbott Racing.
Mind you I spent £2300 on my rebuild, but it did include a LSD, clutch and all the syncromesh hubs!!
Try searching for the Gearbox no on ebay - I saw a 2nd hand FM55 507 for £125 - trick is to search for "saab FMnn" (where nn are the matching numbers on your gearbox), that normally finds all the suitable gearboxes.
Last digit (7 in the example above)is a revision, normally you can use a later number if you can't find the exact match.
Antony.