There are two ways you can go about this. First, try Inventor's built in Design Accelerator. This is by far your best bet. Not only will it generate the gears for you, but it will give you the optimized ratios and force calculations.
If for some reason you find the Design Accelerator unsuited for your application, go to
SDP-SI.. Their CAD Library is extremely complete and I am almost certain you can find what you need there.