There are two major 9mm AR types: Olympic and Colt. The Olympic uses more proprietary parts, including magazines.
Colt-type 9mm ARs use a magazine that is based on the Uzi magazine. The original OEM mag for Colt was made by Metalform, and is almost totally Uzi, except with the addition of a last round bolt hold open feature. Other manufacturers include ASC and CProducts (metal mags) and Promag (plastc). And there are always modified Uzi magazines.
There are basically two different Colt-pattern magazine adapters, too. One is "bottom load" and the other is "top load." Top loading magazine blocks have the advantage of not being likely to fall out, even if you weren't particularly careful in installing them. But they take more work to install and have no adjustability to fit your magazines (which is sometimes needed). Bottom loading blocks need to be secured (most do this with a friction block), but they are usually adjustable to some extent, and are easy to install and remove. I have a Rock River bottom loading block that works with Uzi, Metalform, and CProducts (the old CProducts) magazines just fine.
Hope that helped.