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They're 8 Dromida wheels i had laying around (Hard to see in this picture but it's a dually)
Any wheel around 55mm diameter by 24mm large with a 4mm center bore will do. You could also print some if you've got some TPU or ninjaflex i guess
I will admit the wheel system is not the best for any kind of prolonged use.
I'll try to come up with something with bearings.