SDP-SI has CAD models of pretty much any common gear size you could need.
If you need an odd-sized gear (such as something with a non-commonly available number of gear teeth), get a copy of Machinery's Handbook and look at their section on gear tooth profiles. A few years ago I used this to draw up an approximately correct (I simplified the tooth profile to improve SW performance), equation-driven gear tooth SolidWorks model. Using equations, I set the number of teeth, diametrical pitch, pressure angle, face width, and bore options and it automagically generates the gear model.