Flying Saucer Wheel!?

Why are flying saucer wheels so hard to come by? The metal ones I mean! I've looked all over and can't seem to find a genuine website that sells them. I've found things such as a running plate on Amazon (linked below) but I'm not sure if I would prefer them to use this over a flying saucer!



If anyone knows of where I can get a metal flying saucer from, or if they have experience with the running plates then I will order one of those instead.



Thanks!