Parts for home dish washing appliances can be purchased direct from the manufacturer of your dishwasher, or from third party home appliance part retailers such as Appliance Parts Pros, Sears Parts Direct or Easy Appliance Parts. Parts can be ordered online or over the phone.
Most major appliance manufacturers have a parts catalogue, complete with schematic drawings of their appliances online. Be sure to obtain the model number and serial number of your dishwasher before calling or searching for parts. The model and serial numbers are typically affixed to one of the edges of the door of your dishwasher.
Many manufacturers offer warranty coverage for their appliances. Make sure to call your manufacturer to check and see if part replacement can be covered under the warranty before ordering a part yourself.