Have you tried AIM surplus?
http://www.aimsurplus.com/acatalog/Receivers.html

They probably have receivers, but my friend told me that they are only taking telephone orders now due to the volume in sales. I bought a pair of Superior Arms stripped lowers last year with CMMG lower parts kits- went together easily, good receivers.

Edit: oops, sorry, just noticed that tey are out of receivers too[]