What are these called and where can I get a set?

Thanks for the referrals.

Mine are made, IIRC, of thicker material than either of the other two items mentioned. Once you buy a decent set of casters, material, figure out how to make them and required offsets to keep things level, you'll be in a few hours minimum. Then go to the actual build, it's not a 5 minute job to drill and get proper set up. What is your time worth. Last person to contact me took almost a day to get them correct on their own.

Mine are less expensive than the others and are stout. There is an ad somewhere on the site for what I build and positive comments from those that have purchased.