Springs are cheap.
http://www.brownells.com/magazines/rifle-magazines/magazine-parts/magazine-kits/ar-15-m16-high-capacity-magazine-rebuild-kits-prod25018.aspx
And they are of course out of stock. This is getting ridiculous. Does Brownells have anything that is actually IN stock? We need to find an alternative - a rectangular spring that’s about 18x45mm of the proper strength and length or longer but intended for an application other than a rifle magazine.
Springs for these types of magazines can be made with music wire. Bend it ONCE around a form with the correct pitch and you’re done.
Gunsmiths make springs all the time.
If you need to make a flat spring, then get some 1070 or 1095 spring steel stock and learn how to heat treat steel. Hint: Learn how to use saltpeter salts heated to a liquid state for your tempering. Blues and tempers the spring all at one go.