Hard to find good ones for that size. Actually, maybe you should think about using different kind of gear for this robot. I've seen one german company producing CO2 motors(with mass less than 1 gram inc. co2 supply, )... but that is a kind of solution only suitable for perverts.
Good ones for these sizes are solenoid-type actuators or piezo-actuators. First ones are easy to create/obtain, good linear motion, but power consumtion is too big for a gadget of that size (if you willing to go for minute-two of autonomous work). Piezas... well, whole different story. Pretty hard to obtain suitable one(and they cost a lot), but virtualy they don't use any power at all with pretty good torque and damn small mass...