The Colt magazines made by Okay Industries are ultra reliable and will last for years. I have a Vietnam era 20 rounder as well as quite a number of pre ban 30 rounders (living in New York necessitates this for high cap mags) that run perfectly. You should be able to find these in military surplus stores. If the base plate says Colt or Okay Industries, you are good to go. Just check the feed lips and overall condition and you will find these military magazines will give you years of great use at tremendous value.