Retailers such as Appliance Parts Pros sell replacement wheels for a variety of refrigerators. As of 2015, for example, roller assemblies for Whirlpool refrigerators are available for about $30, and cab rollers are available for about $13.
The owner's manual for the Whirlpool refrigerator should contain information about replacement parts. The customer service department at the retailer should be able to identify the correct wheel based on the manufacturer and model number in the case of an older refrigerator. In many cases, the owner can use the original fasteners to install the new wheel or roller set or use the fasteners that come with the new kit.