There is a standard for steel shelving size and construction, with heavy steel angle uprights with 1/4" holes at about 1" intevals. 'Standard' at least in that I've seen a ton of it in warehouses, etc. and it's carried by a lot of hardware stores. I'm sure most know from that brief description.
Oddly enough, I have not been able to find anyone who makes wheels for it. Even some of the warehousing operations could use that. It would entail only a regular caster-type wheel with a fitting to accommodate the steel angle. My guess is that the product does exist somewhere. Anyone know?