SAS (PA7/PA8) is the pump-action, LAW is the semi-auto. I've never encountered a LAW, but I was told it's mechanically similar to the SPAS-12 and shares some parts. The stock on the PA8 seems very close to the one used on the '12, so perhaps some parts would interchange?
Numrich is quite good for SPAS-12 parts. There also used to be a lot of parts on Gunbroker. Of course getting them exported can be very difficult.
I think there was another outfit that was reproducing a few parts for the '12, such as the recoil buffer. Can't for the life of me remember what they were called.
I'm in the UK and have a SPAS-12 and '15. Never had any luck finding parts for either in Europe, except for a few German dealers that do still have magazines for the '15. Need the fixed synthetic or wooden stock for the SPAS-15.