The S.W.T. UMAG is for sale ($24.95) in the USA. Made by Astra, the same people making quality Swiss made AR15’s, known as the StG4. these polymer magazines look a lot like the polymer parts of the rest of the rifles, and I suspect those parts will make it to the United States eventually as well.
I now wonder though, how many manufacturers of AR magazines are currently available?
Astra SA [Switzerland] & Astra Defense [Hong Kong] astra-arms.ch
Swiss Armament swiss-armament.com [USA Reseller]