I doubt you will find just the M16 lower. Besides, when you're looking for a $8,000 to $15,000 gun, the $500 the upper is worth is negligible.
BTW, the terminology for legal civilian full auto weapons is transferrable, pre-86 dealer sample and post-86 dealer sample. Essentially, you are looking for a transferrable M16. Not only do you get to ignore the "semi-automatic assault weapon" evil features, you can ignore barrel length, overall length and can even remove the stock and make a full auto pistol out of it.