Whoops, my bad! I didn't catch that it was a 9mm. Yeah, when you deviate from the milspec design, then all bets are off. It wouldn't surprise me at all if only RIA mags work.
I did some looking around the interwebbythingy, and you're right about the 9MM aspect changing the game.
I bought some Wilson Combat ETM mags because they were highly recommended by reviewers. A little expensive, but I received a nice discount and low shipping cost from Midway USA for my birthday. Those are at least as good, if not better than, the Sig mags the 1911 45ACP came with. I went for the ETM base mags because those are almost identical to the Sig base.
I was checking out the same mags in 9MM because of this thread. Reviewer said the follower has a notch that can cause the round to not level out when feeding and jam. Not a problem with the 45 round, but the smaller 9MM sits lower in the notch. That only causes problems with the last round (next to the follower).
Anyway, I thought I'd share that. Moral of the story is: not all 1911 45 mag designs will be equally good in a 1911 9MM.
For a RIA 1911 9MM, the most positive comments are for MEC-GAR brand. Second place looks like Metalform, which they say many Colt 1911 have.
One thing I'd recommend is compare the base on the new mag to the original. It may or may not sit flush with the magwell, or it might extend below the grip more than you want..
Second tip: if you might buy another 1911 9MM, you will want interchangeable mags that work in both. So, if that's a potential future buy, don't load up on mags right away.
Go to the gun store websites, read the customer reviews, and don't trust that the most expensive mag is the one that will be the most reliable in your gun.