Airsoft Panda's Zeta Lab just released their own grenade launcher for AK airsoft guns.  This is a full metal GP-30 is a more stripped down under barrel launcher that was designed for use in AK-100 assault rifles. This costs US$85.00 at Airsoft Panda and comes with its own quadrant sight and a 40mm airsoft grenade shell is also thrown in as part of the package.