Gears like this will work:
http://redwolfairsoft.com/redwolf/ai...d_Gear_Set.htm
http://redwolfairsoft.com/redwolf/ai...e_Gear_Set.htm
These modify "drop in" gearsets are also self shimming (AFAIK though if one part breaks the whole unit is toast, whereas with other gearsets you can just replace that one part that breaks):
http://www.airsoftpost.com/product_i...ducts_id=30879
http://www.airsoftpost.com/product_i...ducts_id=30876
PS: Pick the appropriate bearing size for your gearbox/gun, ie. 6mm, 7mm, or 8mm like how the stock gearbox is.
Also Guarder is a pretty good brand as well:
http://redwolfairsoft.com/redwolf/ai...i_AEG_V2_3.htm I run some Guarder stuff in my gun.