Hi sgj3030!,
You only need to purchase the "Lower" receiver from a FFL dealer. It is what the BATF (Government) considers the actual "Rifle"(the part with the serial number)... The rest of the rifle can be purchased on line or at any store that sells the parts.
(I hope the following is OK with the mods
I would also look in to
AR15.com as a source for any, and all information on the AR-15 family of firearms....and a whole bunch more.
Take Care!
Tall Shadow