Bring the broken spring to a hardware store and start shopping! (The only places I know of that might have extension springs that small are ACE Hardware and Menards, but those places are probably only in the U.S.) You might have to find them online, but I never managed to identify which kind of spring it was. Some Google research on spring types/specs will help you out.
Some have also reported success with dental rubber bands. Basic rubber bands will not last and will have the same problem again shortly.
Also, putting silicone grease on the ball valve should help improve its movement, which reduces wear and tear on the trigger assembly.