Caspian (see forum here below)
Essex (Brownell's gottum)
Les Baer (Gil Hebard has at wholesale)
Of course, the frame is the serial numbered major part and is therefore treated like a whole gun. It has to be ordered, transferred, and registered by a FFL. Look them up at Caspian and Brownell's sites and get a Hebard catalog, he doesn't www; make a choice, and have your FLD (Friendly Local Dealer) place the order.