I just spent about 15 minutes looking and found that you're right ... they will be pretty hard to find.
I did find this
http://www.midwayusa.com/product/896...70-long-action
But it's pretty pricey and calls for a bunch of fitting (according to the one written review in the link). It also says it doesn't include the magazine, so probably no spring either.
I'd say you could probably get a gunsmith to fashion a spring for you that would serve the purpose, but waiting until it breaks will have you without the gun whether you're waiting to have a spring fashioned or whatever.
I'd take the gun, whole and unbroken as it is now, to your gunsmith and let him know your concern. He'll probably give you the best advice possible about either acquiring the part in question or about how he can get one made for you.
Good luck.