Dark Eagle Customs makes 10-round mags that utilize factory bottom metal. $95ea though. I think SSS makes a 9-rounder as well.
Member zRat used to make customs using H&K G3 mags, but new laws in Colorado made that stop. Here are his instructions if you want to take a whack at it
http://www.ar15.com/forums/t_6_5/421...formation.html
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