Thanks, those are exactly what I had seen before and were looking for. They do look good, but that isn't the reason that I want them. They will go on my secondary rig, which will go under my L shaped desk, in the corner, which means that it will have to roll over the hump of the leg bases. I had a plastic rolling stand, but that only lasted until I tried to roll it out. Apparently, a roller got crosswise and broke off. This looks far more durable.
Where ever I saw them before, they also had them in black, but this puts me on the right track.