Pen springs have been the standard backup in paintball guns for decades lol. You may have to trim one down to get your desired tension but if you look hard enough you'll find one that fits.
ETEK 3LT, with 14" Ti boomstick and cure 3 + bolt F/S
Vanguard creed eyes, membrane, bolt pin, detents, gaskets, schraders, orings and more F/s[/color]
Old school angel poppets/cup seals(LCD and A4/speed), spring detents(not sure which models) F/S