Stock rims, or wheels, refer to the size of the wheel that came on a vehicle from the factory. "Stock rims" can also refer to the material and finish of the wheels. Aftermarket rims are often constructed of different materials and come in a variety of sizes for most vehicles.
Rim size is measured in inches and is separate but related to tire size. The size of the rims can impact fuel economy and handling, especially in the case of oversized rims. You can purchase factory stock rims from a dealership or manufacturer of the vehicle or sources such as StockWheel.com and AllFactoryWheels.com.