I bought the toothed idlers from Printed Solid for my 2nd MK3. I expected they might not fit given they were wider and, sure enough, they are just a bit too wide. So I modified the original Prusa part to accommodate. It's really the same part - just with a bit taken off with simple CSG in TinkerCAD, though it appears to do the trick.
(Note I'm not done assembling the printer yet to verify but the part does allow free movement of the idler so it should work, but your mileage may vary).
I printed mine in Atomic Filament PETG with a .2mm layer height and 100% infill. You could otherwise use the recommendation from Prusa for printing parts as this one is so similar there isn't really any cause to print it any differently.
The author hasn't provided the model origin yet.