Ok...I have owned 4 AUGs and currently have 2. The simple answer would be to go with the NATO stock. You have the mags for it already and the gun can even use higher capacity ones like the 50 round drums and the 60 and 100 quad stack mags. Yes the standard AUG mags are stll available at gun shows but tend to pull a higher price than a typical magpul mag.
The NATO stocks are available out there from either PJs AUGs or on the firearm sale sites like gunbroker. PJ also sells AUGs with the NATO stock already installed but they arn't difficult to do yourself.