im making a airsoft trip mine
I got a good strong spring from a nerf gun (not to long though).
Try Shortyusa.com they sell springs there try the m120
Clynder head valvue springs.
Alice Springs! Sorry, i've always wanted to say that. Try... eBay. You can find virtually anything there, including portions of the sun (i'm being serious, someone tryed to sell the sun on ebay.)
Torsion? Compression? Flat?
First stop would be a real hardware store, they'll have a spring assortment. Next would be to look up spring manufacturers or spring suppliers in the yellow pages-but you will have to know what you are looking for, as in how long, what type, how strong...