both my 9mm & 10mm hold enough rounds, & the gun / ammo combo is heavy enough that I don't carry a spare mag... if 15 isn't enough... I shouldn't have been there...
One spare mag on my belt opposite my H&K P7M8 in a Horseshoe Leather MV1 (the gun rides in his MLS), or a speedloader riding in a Safariland metal clip in the same position opposite my Taurus M85.
When the M85 is in my front pocket, where I usually carry it, I have the speedloader in the other front pocket or a jacket pocket.
Don Hume (donhume.com) offers a single mag carrier that works pretty well. It clips onto your belt and is no harder to conceal than the gun you are carrying. The price is between $20 and $30. Not a bad deal and its put together pretty well.
I carry a spare mag in my left front pocket.
I use an Uncle Mike holster with the clip removed. It holds a double stack mag pretty good. And keeps it from shifting around to much.