Good luck with finding an off-the-shelf spring; Several parameters need to all be correct:
- rest length
- diameter; at least an upper limit to fit in available space
- most important: stiffness; that depends sensitively on the wire diameter (all other things equal); like the cube of the diameter IIRC, so that diam_x2 means stiffness_x8.
Most likely need to roll your own; see:
http://www.deansphotographica.com/machining/projects/springs/springs.html
This guy is a perfectionist, I get away skipping some of the steps. And I don't have a lathe, I use a hand drill fastened on the table, leaving one hand to turn the crank, and the other to guide the wire.