They're auction sites, so you'll have to register first. The prices are usually better than other places. Hi-caps are still readily available for the 6906, and rather cheaply as well.
I suggest www.cdnninvestments.com. I bought 2, 13 round factory mags from them (I think the price was $29). What I really like for this gun is the 59 series magazines with an adaptor. Puts about 1/4" on the butt of the gun and gives you 2 extra rounds (cdnn carries these with the adaptor also). Sadly, the adaptors are no longer available (you have to get them with the magazine. I get my supply from a local PD that issues the 6906. Gun shows are a good place to find mags also. I absolutely love the 6906.
I saw one at a gun show this past weekend. It was AN and had a proce of $15. I had my 5904 with me but the mag wouldn't fit - I knew it was a 14 shot deal. Also bought 2 10 shot mags for the Colt AR 7.62.39 for $5. ea. Sometimes it is not what you sell but what you pick up.
mac