I have a DelTon lower bought from them and shipped to my FFL, completed the build with all DPMS parts (complete upper, buttstock, lower parts kit) from Midway USA. The problem you’re going to have right now is finding the lower receiver (from anyone, let alone DelTon) and parts kits.
Apparently, there are lowers out there again. I just did a Bing search on ar-15 lower and got three sites selling them. Their prices range from low $200s and up.
Never heard of this place, but they have several in stock. http://www.carricozarmory.com/lowers.htm