Depends what kind of wheel you're using.
If it's got bearings in it, and you're driving the wheel, not the shaft, you can use any standard shaft (The most common is 3/8" I believe)
If you're driving the shaft and not the wheel (ie: the hub is keyed) you need a keyed shaft of that size
Either kind can be found at
www.mcmaster.com
P.S. I'm moving this to the technical forum.