There are three pretty decent options available:
1. As mentioned above there are some companies that make some pouches that slip on your belt. The leather variety that with a snap flap look a lot like leather knife pouches, so they do not attract any significant attention, especially with a stainless magazine that looks like a Leatherman from a distance.
2. In that same line of thinking a leather or nylon knife pouch that is the right size for your magazine works great, and will often even say "Leatherman" or something similar, further creating the illusion you're carrying a knife.
3. And as indicated directly above, an open topped cell phone case can work well to allow you to carry one or two magazines in your pocket without letting them fall over sideways in the pocket. Just be sure it's "sticky" enough to stay in the pocket when you draw one of the mags out of it.
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There are also some tuck able IWB magazine holders available (Wild Bill's Concealment sells some), but generally speaking carrying a magazine IWB, and in particular tucked under your shirt tends to be uncomfortable as it digs into your side over time.