Tactical Vantage has some Steel and Polymer AR mags in stock. They even has some promag 42 rounders. Ive never used the 42 rounders, but youtube had a couple good reviews of them.
Steel 30rd-$18
Polymer 30rd-$15
Polymer 42rd- $18
http://www.tacticalvantage.com/en/381-col-a18-ar-15-223-20-round-magazine-black-polymer.html
Steel 30rd-$18
Polymer 30rd-$15
Polymer 42rd- $18
http://www.tacticalvantage.com/en/381-col-a18-ar-15-223-20-round-magazine-black-polymer.html