You're looking for a recoil spring. Illusion has a few I believe:
http://www.airsoftcanada.com/showthread.php?t=495
Specifically the
Shooters Design Metal Piston Head Set for Marui Hi-Capa Series
I would go that route first, but if you feel like a trip out to Princess Auto, they sometimes have a 100 Piece Spring kit full of a mish mash of springs. You might find one that works, or you might just crunch your nozzle... (again?)