Depends on what you want. You will probably have better luck with a 11.5" that a 10.5" For some reason, the short .223's are 11.5", while the 9mm's are usually 10.5"
I would probably lean toward Colt or Bushmaster. Or go your route and build your own which is always fun too.
Possible sources, JT Distributing, Bushmaster, Rock River, Model 1 Sales, etc. Avoid Olympic Arms. Right now, this stuff is still unobtanium for the most part.
There is a fellow named Dennis Todd that has a ton of factory Colt parts. Hard to beat Colt and Dennis is a good guy. DToddmgREMOVETHISSPAMGARD@AOL.com.
I've done several builds using stuff from JT Distributing. OK stuff, not super high quality, but very functional. Have a 11.5" parts build on my M-16. No-name A3 receiver and Bushy barrel, runs flawlessly and is accurate.
One that gets good reviews is from POF-USA. Very pricey, monolithic design, gas piston.
http://www.pof-usa.com/upper/upper415-6119X.htm
And of course, you could spend mega bucks ($3,000 plus) on an HK upper or one from Knights Armamament if you can find it.
Don't forget, if you are building a rifle with a barrel under 16", you must have in your possession a registered NFA lower before you purchase the upper. Either a Short Barrelled Rifle or a Machine gun. You can build a pistol from a virgin receiver, then install the short upper and be legal. Just don't get caught with an AR-15 and a short upper. ATF considers it "constructive" possession of an unregistered device. Gets you an invitation to Club-Fed for 5 years or so.
Ken