This is a 2.5" diameter x 1.5" wide R/C car wheel.  You can get them at your local hobby shop, or as part of the Wheel1 Kit from Solutions Cubed.

This is the aluminum hub you need to use to mount the R/C car wheel to a 1/4" shaft.  With one of these hubs you can bolt a wheel directly onto the shaft of a Hsang Neng motor from Jameco or Solutions Cubed.  Solutions Cubed sells this hub as part of the Wheel kit above.

This is a rubber,  1.25" model airplane tail wheel, with aluminum hub.  I picked it up at the local hobby shop.

This is a AA sized, rechargable battery cell.  Put several of them together to form a battery pack for a robot.