I don't know if this has been posted before but i have been buying my mags from http://www.botachtactical.com/ and have great luck.
Cheep and free shipping Just wanted to share the wealth
$21.98 for Smith & Wesson M&P Pistol Magazines
Cheep and free shipping Just wanted to share the wealth
$21.98 for Smith & Wesson M&P Pistol Magazines