Besides the factory mag? Looking for reliability at less cost.
Metalform/Spikes
but those arent much cheaper than Colt.
You can try C Products, some guns run them some dont. My build w/ Hahn dedicated block loves them.
But then direct from Metalform.
BTW Metalform made the factory colt mags for many years.
In the last year or so CProducts has made the mags for Colt.
CProducts are a lot cheaper and a classic example of you get what you pay for.
This is a pretty good price for Metalform mags: http://www.cdnninvestments.com/ar9mm32blmag.html 
If you can live without the bolt stop being activated, modified UZI magazines are superb.
J&G has some UNmodified 32 rd UZI magazines here that can be converted to work in the AR by cutting two notches in the magazine tube. I notched mine in a manual mill but I’ve also seen a video demonstration somewhere on the net of a guy rough cutting the notches with a Dremel and truing them up with a file.
Wildcat
Spikes are made by Metalform, so they are essentially the same as factory colt mags. I had trouble with my C-products mags so I sold them dirt cheap and bought some Spikes mags.
I always just used the factory COLT mags, and NEVER had a problem. I figured it was the tax on getting a reliable 9mm AR system.
I’ve heard good things about the Metalform mags, but still chose to buy what I knew would work 100%.