That's how we used to get the Oly Arms kits, but now that "buildin' an AR" is being done by everybody and his brother in law I think the uppers are being sold assembled and headspaced for liabilty reasons. It also removes the need for a specialty bbl nut wrench, the rest can be put together with some punches and a screwdriver. I haven't seen the complete unassembled kits in years.
I would call up the vendors and ask them if they can sell you what you want as a package deal, it just might be they don't see a market for a "Bag o' Rifle parts" and don't list it. Otherwise you could buy an assembled upper and a lower kit then disassemble it. Probably be cheaper that way than buying all the parts individually.