SRAM cards are getting very hard to find.  Envoy Data will be your best bet.  Your next problem will be finding a computer and card reader that will read the SRAM's.  W2K and XP are very difficult to get to read the SRAM's.  Some machines with these operating systems will read them and some just will not.  You might be better of finding a PF3000 head to put on the combine.