I've seen them before. They just look like straightened out AKM magazines.
edit:
Ironically from the site I was trying to find. Here is a list of AK mags currently, or that have been in production in the past from various countries. Fun read!
http://home.comcast.net/~shooter2_in...mag_guide.html
I've seen some people mod real AK mags to fit for airsoft. So with a bit of effort (And whether its legal or not?) you could prolly have a project ahead of you.