There's about any option, caliber, length, etcetcetc. BARREL has to be 16.1" min or you get to apply for stamp. Here's a link:
http://www.ar15.com/forums/f_3/4_Build_It_Yourself.html
You will probably have to sign up for membership but there's also most of the mfgr's on this site too.
Goo luck, uppers are only a little more diff to build than a lower. Probably takes longer to figure out/find what you want