may be able to shove an M9 spring or other in there if you cant find a replacement, otherwise, a rubber band from the trigger around the front of the trigger guard will work in a pinch, ive done it before, but it was an AEG not a GBB, may have trouble resetting the trigger
http://sell.lulusoso.com/selling-lea...48-R6729-.html
not sure if that link is any help