Importing guns and gun parts is a complicated legal process, requiring an FFL, an Import License, and lots of fees and paperwork. Though these are just "parts" in the US, they are treated very differently when crossing international boarders. And Customs has NO sense of humor. But, with a bunch of effort, and not a little money, you could get it done.
Anyway, thanks to Klinton's 2000 EO, you can no longer import receivers or barrels for anything "assault-weapon"-related. I'm not aware of anyone making these parts domestically.
-Troy