The Springfield armory store always has mags. Prices are so-so. I order from there took about a week. Check the link below.
http://www.springfield-armory.com/store/index.php?main_page=index&cPath=46_55_70
I have 2 and love them. Highly recommended. Feels like it was made for my hands. All my new builds will have this grip. After you wrap your hands around it, you will understand.