What we really need are published templates to cut and form sheet metal into magazines. Some company could sell tempered springs; I doubt there is a law against springs, yet.
Millions of men have the skills and the tools in their garage to make steel mags. Just buy 10 round mags, cut, splice and weld together. Spot weld the springs.
AR 30-Round Magazines Now Available for Download, Seriously
http://www.docstoc.com/docs/141644816/AR-30-Round-Magazines-Now-Available-for-Download-Seriously
You can make your own springs by wrapping piano wire around a dowel
Springs are nothing but mandrel bent piano wire.