I picked up one of these suppressor pouches recently and MP9 mags fit well in it. Not snug enough to run around with them open though.
http://www.airsoftgi.com/product_inf...oducts_id=7046
I'm shopping for MP9 pouches right now as well. I really like the suppressor pouches. They're well made and the lower half of the buckle is attached to the pouch so it doesn't dangle. However I think velcro would be faster, and I'm leaning towards the triple MP5 pouches.