I made a pretty large purchase from PCB with the monolights and "stuff". I'm thinking I got the casters at that time. You could probably find them cheaper elsewhere, but they do fit properly on the legs of a light stand. The casters are female and the stand legs are male. This would be different for a caster that has a mounting plate.
One note of caution: if you put casters on your stands, make sure the wheels are going in the same direction before you make any quick maneuvers, especially if the stand is holding a heavy monolight and heavy modifier. I usually make a small semi-circular move to align them before repositioning.