Cast your own AR lower. Just drill and tap. Looks like one lower per mould. Could be fun and it's on ebay.
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/232884823872
https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/232884823872