Here are some wheels that would be pretty direct replacements:
IFI traction wheels (6" x 1"):
http://www.ifirobotics.com/robot-traction-wheels.shtml
AM Plaction Wheels:
http://andymark.biz/plactionwheel.html
AM Performance Wheels:
http://andymark.biz/am-0132.html
Other AM Wheels:
http://andymark.biz/08firstwheel.html
http://andymark.biz/firstwheels.html
These all have the same 1.875" bolt circle, but the sprocket mounting and bore holes might differ, so it depends on how your wheels are driven (sprocket, direct drive, idler etc.) Make sure before you buy these that you look into things such as the diameter of the holes on the bolt circle.
Something else to keep in mind is whether or not you will be able to turn when you put on traction wheels. A simple 4 wheel robot will turn fine with rover wheels, but with 4 traction wheels it will have a lot of trouble.