As far as I remember, Cabelas had the kits and spare mags not too long ago. The kits were under $200 and the mags were $25, IIRC.
Unfortunately, they are listed as out of stock on their website. I checked at Cheaper than Dirt, Natchez, and Midway, too. No dice.
Ciener sells them directly, but the whole kit is listed at $200 and the mags at $40.