I am always on the lookout for robot things I can get cheap... I just happened to stop and check out the "AB Wheel" thingie at Wal-Mart on sale for $3.00 each. You get two wheels with each "Ab Wheel" exersize thingie. The wheels are nice plastic ones with a hard rubber tire tread, with what looks like a 1/2" or 5/8" axle hole.
It has two approximately 7.5" diameter light weight wheels with .9" axle holes in each kit. Thus if you buy two of these you get four wheels for $6.00 plus tax. Of course if you use a smaller idler wheel(s) you only need one kit with two wheels for $3.00. You can also double up the wheels (dualies) if you like too. So the big robot wheels only cost $1.50 each in this exersize thingie. :)