if you are really really stuck I got one with a few small cracks round by bolt release that "might" be ok with say some resin or glue to ensure it lasts or don't develop any worse. It is painted OD but say a fiver posted if it is any use to ya
thing is you might just be better off buying up a few cheap metal receivers from GF or TWG or HongKong in the end
http://www.taiwangun.com/en/metal-parts/asr-upper-and-lower-receiver-black-aps?from=listing&campaign-id=14
the thing is, I've already got he P* installed in my CM and I just want another upper to switch roles, don't fancy getting a whole new receiver
buying a full receiver won't effect the P* buddy keep it in the same gearbox and drop it into another receiver