http://www.ebay.co.uk/itm/Moto-Guzzi-California-Seat-Cover-All-Versions-New-/260730641001?pt=UK_Motorcycle_Parts_13&hash=item3cb4c1da69#ht_551wt_1157 \
I bought a V1000 one it fitted fine.
Not used this one below but they look interesting
http://stores.ebay.co.uk/TOP-GAITERS-BOOTS/MOTO-GUZZI-LEATHER-SEAT-COVERS-/_i.html?rt=nc&_fsub=3887984016&_sid=668506186&_trksid=p4634.c0.m14.l1581&_pgn=1guzzibear2014-02-02 21:32:25