If you ever so happen to find yourself in the unfortunate situation of needing to replace your toy wagons wheels, you can easily find them online at Global Industrial, Service Castor Corp, Amazon, and Radioflyer.com.
It may also be useful when replacing your wagon's wheel to search on Youtube for instructional videos, or the owner's manual that you received upon initial purchase of your wagon. At times, along with replacing your wagon's wheel, you may also need to replace a part or piece of the axle that is crucial for the wheels of a wagon to function smoothly. Be sure to look at both the wheel and the axle when determining the problem.