I'd guess the best way is to adjust some spring that is "close" enough. Stretch it, clip off a couple turns, or add some spacers and you should have something that is close enough. If that isn't possible, try replacing it with something else - thin rubber tubing is sometimes used in place of springs, with the last solution being to make your own - music wire, drill bit, vise and some locking pliers and you'll have something that is functionally equivalent.
I've never seen assortments of small springs - my local Aces hardware has some of the larger sizes, but considering the size of stuff in most camcorders, I doubt any retail store stocks the stuff. I think there are some distributors which deal with VCR repair parts, but I doubt it would be worth it.
James