As someone else pointed out they're rather easy to build.
My Colt M16A1 came with the Vietnam-era A1 furniture, which I sold rather quickly on GB so I could make mine into a de facto M4.
So they're out there, you just have to look. You may want to post on
Sturmgewehr or
Subguns.com where quite a few people have M16A1s, but no use for the A1 parts and are willing to let them go. Then you can just slap them on a regular lower and have a rather close copy.