Midway has the mags:
CMMG 25R3 Rimfire Conversion Mag AR-15 CMMG Ciener Tactical Solutions
For what it's worth this design has been around since at least the
80's and the bugs are worked out of it nicely.
I've got a couple of the Atchisson kits which are earlier versions of
this kit that I use on my M-16. They run fine and CMMG are as good
with the exception the CMMG kit doesn't come with the full
auto trip and anti-bounce device needed for full auto fire.
Maxwell Atchisson designed a good conversion.