You can check GB and even E-bay. It's basically an adapter package that can go is any AR15. Package usually includes the BCG replacement unit and adapter mag.
Just Google "USAF .22 adapter" and you'll get all kinds of hits.
I'm sure Primary Arms, DSG Sales, CDNN Sales, JSE Surplus, and your favorite ammo place Midway ? will sell what you need in a package deal.
Unless of course you want to buy a dedicated weapon the adapter is the way to go as I'm told. Most 1-9" twist (and slower) AR15 barrels are perfect.