I bought a Colt M16A1. It is at my dealer's shop and I'm waiting for my paperwork back from ATF. When I went to look at it I noticed that the ejection port cover doesn't latch as it is missing the little detent ball and spring. Where can I get replacement parts for it?

Probably best to replace the whole cover.  They're about $12 or so, or approximately 1/1,000th of your likely investment.

~Augee

Last time I checked, you had to debarrel the upper to properly replace the port cover... at least I've never had much luck trying to remove or insert the rod from the rear, and in front it's blocked by the barrel nut when the barrel is installed to the upper.

That might make it worth his while to just replace the detent, spring and retaining pin.  

Since it doesn't seem like any of the major mfgs carry just those replacement parts, though, it may be necessary to obtain the entire port cover and cannibalize parts to repair the existing port cover, if azjeffs doesn't want to remove the barrel nut in order to properly replace the port cover.

You use a fine pair of pliers to pull the TINY "c" clip that holds the pin at the front, then you pull out the pin and take off the door.  When you put the door back on, it's pretty easy to get everything lined up right and the door back on.  Once the pin is back in place, use strong light and a magnifying glass to reinstall that "c" clip!

It's not as big a hassle as it seems, it just takes a little patience.  And a magnifying glass.