I have ordered BCM lowers from Rainier Arms and Primary Arms years ago and had them FFL transfer at local gun store. Looks like Rainier is out but can "notify when in stock" at the moment. Primary Arms will allow back order them.
I have bought a lot from SWFA and Grab-A-Gun (but not BCM lowers) and they both list BCM lowers out of stock but can "notifiy when in stock".