Do you want ever part as in barrel, handguard, receiver, ect. Or just an upper parts kit like the Dust cover, forward assists, ect? The fun of building an upper is picking what you like, because buying them done is cheaper and easier, so if theres something specific you are looking for, you can build it...
http://www.surplusammo.com/products/DPMS-Upper-Receiver-Parts-Kit-w%7B47%7D-Charging-Handle.html
Free Upper receiver with barrel purchase.... they have a few barrel option, from hbar, to bull http://www.surplusammo.com/blackhole-weaponry-ar-15-16-m4-stainless-steel-5-56-1-8-poly-barrel/
Armalite bolt for $100, you cant beat that.. http://www.armalite.com/ItemForm.aspx?item=EB0170&ReturnUrl=Categories.aspx?Category=e92d134a-2fae-454b-a1cd-eb398588add2
SAA Has $300 DD free floats for $200 also... They have the black gas rods for $15, then pretty much all you need is a M/B or what ever you want at the end of the barrel...
good luck