Each maker has thier supplier for mags, some even make thier own, then there are after market makers that you can get. which will work best can be a trick to learn because pistols vary and what works for one brand doesn't always work for another. Good luck on which works best for you.

Who sells what with their guns is subject to change and generally speaking you're going to end up buying aftermarket mags any way, stick with Wilsons, Cobra Mags, ACT mags, or McCormick Power mags and you should be good to go.

I don't know why but it seems like most OEM 1911 mags are crap. I know S&W and Rock Island used to come with ACT mags which I've had good luck with. Colt and Sig have come with Checkmates and Metalforms, I think Kimber does their own now.

Danite,
Good question. Answers may vary, but here's my take. I have a Remington 1911R1. I have heard that MecGar makes their mags. I have a Colt Mustang and don't know if they make their own. Same for my Springfield XD Tactical.
I do know when I ordered spare mags I went to Midway for all of them as they have liberal return policies. Both mags are marked Rem, Springfield, and Colt respectively. They work. I understand Sportsmen's Guide sells 1911 .45 mags., but I haven't tried them as they were out at the time, but they also have a satisfaction guaranteed policy.
My experience is if some supplier is offering mags at a super lo price be careful who you buy from.