If the car part or car parts are something I need right away to fix a problem with my car or someone else's car then I go to an auto parts store such as Autozone or O'reilly auto parts and get the part there if they have it.
But if I don't need the parts right away I buy the auto parts online through
http://carparts.com or if it's something else such as a starter or alternator or other electrical component that carparts.com doesn't have I go to
https://www.dbelectrical.com/ or ebay.
Usually buying car parts online is much cheaper and sometimes you don't even have to pay sales tax and even though you may pay shipping and handling cost it usually works out to be cheaper than buying the car parts locally.