I see the advantage of these over the tapered rollers for a flat bottom boat. The center of the roller would need to mate with one of the strakes on the bottom of the boat. But no, I don't know where to get them, I've never seen one before.
I use one with My Mod-V Tracker. It works well, but the roller and pin have deformed under the weight over time. It doesn't roll as easily as it once did. I would get the strongest metal for the pin that you can find, or use a light boat.