I'm sure Cane has some more (much more usually) sources than these, but I have never had a problem with any used surplus. I got a bunch with "East Germany" stickers on them and they work great. Anything steel from Hungary, Romania, Bulgaria or many others will work great.
I just took all my mags apart last weekend and checked the quality of the springs. Out of darn near 20, only 2 were really compressed. Who knows if it was just well used or stored fully loaded too long, but I just replaced them with the 5% extra strength springs that midway has and filled 'em up ~75% of capacity. Good (or better) as new.
Here's a couple of the economical/consistent sources I use.
Arms of America LLC
Shooting Supplies, Reloading, Gunsmithing, Hunting, Ammunition, Gun Parts & Rifle Scopes — MidwayUSA
Careful with Midway's 40 round surplus. Last time it came in poly. Oh well. . .