There are plenty of new and used Colt Match Target rifles and carbines available on GunBroker. I'm not sure I'd pay the asking prices, but they are available. There are, or were, some actual M16A4 uppers and/or barrels available. If you get a Match Target rifle or carbine, you get a heavy vs government profile barrel. If you don't care for the extra weight up front, you can purchase a government profile barrel (also available on GunBroker).
If you are not completely wedded to the M16A4 platform, I'd suggest getting a carbine buffer tube, buffer, spring, and stock. I like adjustable length of pull for use with optics. I like the MagPul CTR, but you could get a Colt Defense / Rogers Super Stoc if you're obsessed with using Colt parts.
I would make sure you get a "small" pin lower, though. Parts for "small" pin, vs Colt "large" pin lowers are easier to come by. I think Colt stopped making "large" pin lowers in 2009.
Small Trigger Pins: .154"
Large Trigger Pins: .170"
Large Front Takedown Pins: .315"
Small Takedown Pins: .25"
There are various model numbers that Colt uses to refer to very similar rifles that fit the description of what you say you want. The MT6700 is the closest Civilian rifle model to what you said you were after. Several of the ones I've seen advertised were obviously small pin (more desirable) and brand new. To make it a clone of the M16A4, you'd have to find an undrilled Colt front sight block with a bayonet lug and then thread the muzzle for a flash suppressor. Good luck with that. By the time you get done with all that, you'd probably have been better off just buying a Colt M16A4 barrel or upper assembly.
I'm not completely sure which bolt carrier assembly the MT6700 comes with, but you may also have to purchase a Colt full auto bolt carrier assembly. Again, it all depends on how close you want to get to a M16A4.
If it were me, I'd just get a Colt M16A4 upper and then mount it to your lower of choice. Colt makes quality lowers, but they're nothing special in the realm of forged lowers.
If GunBroker doesn't have what you want, there is also GunsAmerica and GunAuction. There are others that I go to, but the names aren't coming to me off the top of my head. Google, or your search engine of choice, is your friend.
Specialized Armament can supply the small Colt parts for your build. Again, a little Googling can turn up little caches of parts here and there.
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