Tons of options. Maxpedition and 5.11 have several 'mag bags'. Everything from a simple fold open deal with elastic bands for the mags to basically small range bags that have MOLLE or mag pouches on the outside and inside, and pouches for accessories and can carry a hand gun or two.
Most range bags being made today generally have a section with the loops for mags. Depending on the size of the bag, it might be 4 mags, or 12.
One style I started seeing a lot of after this years Bianchi Cup and Steel Challenge matches was an interesting 'holster' that held oh, half a dozen mags or so. Open sided, and the mags appeared to be clipped in side by side. Rather bulky and specialized to those matches, but interesting none the less. No idea what it's called or who makes it, but I saw several of them on Pros and amateurs a like, so they gotta be rather 'common' as far as that kind of thing goes.