Tokyo Marui makes a 250 round hi-cap AK shortmag as seen here http://www.combatdepot.com/product_i...0eeac93d65117c
I haven't found any mid or low cap AK short mags however if you find some contact me, I'm interested in picking some up.
The only other way I'm thinking of involves a lot of work and may not actually work which is to take a standard length AK mag and cutting it down while modding the internals to work with the cut down mag. This probably involves a dremel, messing with the feeding track and messing with the spring. Like I said a lot of work for not too much gain and possibly destroying a mag in the process.