Wheel for a operation stand

I recently bought this surgery/operation stand with porcelain tray not sure of the year. Good shape just surface rust but it is missing one wheel, most wheels I see have the post on the wheel not the cart, does anyone know what this style wheel is called

 

.... so the post is welded to the cart? Can't say I have seen that, nor will I think it easy to find. Generally, called casters rather than wheel.

 

Ditto Brad-You may have to replace all the casters to get a match.

 

Often called a Mayo instrument stand.

 

Thanks all, I've never seen them like this either just wanted to see if anyone else has.