As mentioned, getting some existing dimensions (axle length, axle diameter, wheel width & wheel diameter) then searching the Internet goes a long way.
I've used Polyurethane coated aluminum wheels before. They have a decent bearing (long needles), have a grease fitting and work well on many surfaces.
Issue is they're a little pricey, but last longer.

You can also look at:
WWGraingers.com
McMaster-Carr
Harbor Freight