The lower receiver is the serialized part that has to come through an FFL. The rest can be purchased/made/scrounged whatever.
Yes you can buy all the parts sparately and assemble your own.
Just do a search or visit AR15.com
Actually, you can make a lower receiver. The only caveat is that the weapon has to be for personal use. As I understand it, no documentation is required.
My understanding is that there is now law preventing anyone from manufacturing their own gun, including the lower receiver, from scratch. (No numbers needed)