Yes, it is possible to get a Colt with selector stops on the lower if you buy the new Colt M16A1 reissue retro classic series models either available now or early next year.
These classic series models also have the original gov't contract rollmarkings and all features except they are configured in semi auto only (not select fire).
The AR-15 semi auto models does not need the selector stops, the selector are stopped by the groove inside the selector and the selector detent pin, they will only rotate 90 degrees.
The only bad thing about these classic AR series are the price they will be expensive at about $2,499 MSRP or even more because they are limited production.
Here's an article link of the new Colt M16 reissue classic series models:
http://www.gunsandtech.com/news/tyan/first-impression-colts-new-retro-m16a1-reissue/