Another reason why the Franchi SPAS-12 and parts for it is rare is because it was discontinued during the mid-'90s.
Beretta owns Benelli and Franchi (along with several other companies)...
When Benelli introduced the M-3 Super 90, Franchi discontinued the SPAS-12.
The Benelli M-3 Super 90 is a selectable pump-action/semi-auto that was lighter and more reliable than the SPAS-12. That was one of the reasons why Beretta had Franchi discontinue the SPAS-12, another reason was Beretta's descision to have Franchi pull of the Mil/LE shotgun market (so Benelli & Beretta could market it better) and concentrate on sporting shotguns.
Due to it being discontinued and it being used in several movies & video games, the Franchi SPAS-12 & SPAS-15 are collector's items and are priced accordingly.
You can currently get better (reliability/accuracy/handling) shotguns, than the Franchi SPAS-12, for less money.