What gears do you have now?
Are you specifically talking about gears, or are you refering to sprockets?
Typically, you won't find a spoked gear until you go to very large toothed gears. Most teams here will lighten the gears themselves by either drilling holes through the bulk of the gear, or turning down the "webbing" of the gear (or both...) If your design does not require a lot of torque, you can use plastic gears, like those from the Fisher Price gearbox.
Plastic Gearbox
If you're looking for sprockets that are lighter, you can find aluminum sprockets from both
IFI Robotics and
AndyMark.
Good Luck,
BEN