No matter if the wheels have broken, or you just want a spare lying around in case they do break, you might find this useful.
eModels
Tamiya Spare Parts 60D Reinforced tyres A*2 # 53340
Model Bau
Tamiya 60D REINFORCED TIRES A*2 53340
OnBuy
Tamiya 53340 60D Reinforced Tires A x2 - RC Hop-ups