which preban AR is closest to m16a2 config?
I want one that looks exactly like the m16a2 as possible. How much would one of these run?

Colt AR15-A2

where can you find a Colt AR-15a2?
how much are they?

I would say a Pre-Ban Bushmaster would be your way to go.

To get a Colt Marked AR15 you would have to go back to Green label gun and will probably get stuck with non std. Front Pivot pin and slab sided receiver over the re-inforced mag fence of the military A2 guns.

Get a later Colt 6601 and you could either get slab side/ sear block, no bayonet lug,over sized trigger group pins or any goofy combination of the above.

Get the Bushmaster and the only thing missing will be the machining needed to accept the full auto sear.

Aside from the aforementioned screwed-up pin on the front of my gun, I believe the R6551 that I have is close in profile (heavy in front, light under the handguards, A2 sights and flash hider, etc.) to the current issue.

AFARR