This caliber is, as of December 2008, unavailable in Boxer-primed brass as far as the collective knowledge of the internet seems to go. Pretty much all of the cases out there are showing up already loaded, and made out of steel.
However, it is worth noting that this caliber can be formed from 222 Remington brass (this is NOT a typo), however a small percentage of the cases are likely to crack. It's also possible to form it from 223 Remington cases, but a much higher proportion of cracked cases will occur.
It's also worth knowing that the bullets also have to be reformed from .224" bullets normally used in 223 Remington / 5.56mm. 5.45x39mm is actually .221", and at the present time there are no commercial bullets made in this size. Therefore, one must compress .224" bullets until they lengthen out a bit and end up as perfectly round .221" bullets.
-- SeanNewton - 13 Dec 2008