You can get uppers from several places on line. You don't need to go with a "brand name" like Bushmaster, Rock River, etc... and most of these vendors have a number of options that you can upgrade your rifle with.

Here are a few but if you do a search you can find more.

J&T
http://jtdistributing.net/store/more_k_bbk.html

Ranger Sales
http://rangersales.com/index.php?mai...83e20a6ee0c9a9

KB Mfg.
http://kb-mfg.com/index.php?main_pag...2785a5aea418b3

AR15-kits.com
http://www.ar15-kits.com/index1.html

M&A
http://www.mapartsinc.com/productsDetail.asp?id=14

Mod 1 sales
http://www.model1sales.com/index.cfm...ategory_id=171