Nearly any store that sells air conditioning filters for vehicles can identify the correct filters to use. This includes both brick and mortar storefronts, as well as most websites. The owner's manual for the vehicle also provides information on which AC filter to use.
Typically, owner's manuals provide part numbers that correspond to parts that the vehicle manufacturer sells. For example, Ford owners' manuals give part numbers for its in-house parts made by Motorcraft. For more generic parts, a store website such as Amazon.com or OReillyAuto.com allows an owner to input the vehicle year, make and model, and then it generates a list of compatible parts.