I don't think you can buy the adapter at an auto parts store. The adapter is basically a two(2) stepped sleeve that slides on the spindle with the larger diameter toward the inside of the vehicle. The rear seal of the rotor fits over the wider part of the adapter to stop dust from entering. I don't remember for sure but I think the inner rotor bearing stops a the start of the wide diameter of the adapter, thus adjusting the proper relationship the the caliper mounting bracket.
