This X High-Tech CNC 7075 Aluminium Reinforced Gearbox will fit most M4 AEGs, usually the Tokyo Marui-compatible ones. If you're ok with a red-coloured gearbox with what seems to be 8mm bearing and comes with all the small parts you need to complete the gearbox before dropping it in your AEG, you can get this at EB Airsoft for US$119.98, free shipping included.