File this under the category of "Why didn't someone think of this sooner?" Most folks who carry a defensive firearm will carry a spare magazine along with the rest of their daily gear. Some will utilize a magazine pouch, some use cargo pockets and some, like your host, will often just toss the magazine in a pocket.
SnagMag has come up with a very clever alternative to pocket carry with the company's innovative Concealed Magazine Holster. It's a polymer-frame container that slips in your support-side pocket with an unobtrusive clip to keep the reload stationary. Rather than have the magazine loose in the pocket, where it can often get turned around and less-than-perfectly accessible, it's always right where you need it.
While not as immediate as a magazine pouch on the belt, the SnagMag carrier keeps things secure and is available for a wide variety of magazines. It is available for right- and left-handed shooters.