As well as what others here said partsgeek.com has had inventory that rockauto hasn’t for me.

Rock Auto for general parts.

Dealership (or dealer-sponsored discount sites) for parts that need to be OEM.

Rock Auto for stuff you need next week. AutoZone for common stuff you need immediately. O’Reilly for uncommon stuff you need immediately. eBay for used OEM stuff.

Rock Auto is cheap but you’ll have to wait for shipping. AutoZone is not too expensive but won’t have rarer items. O’Reilly seems to warehouse almost anything but is more expensive. And if you have no other choice, NAPA.

Had a starter die on me. Autozone had one in stock in store nearby and I had the car fixed that day.

I had a rat chew some wiring all the way down to the connector on a Saturday evening. Rock Auto carried a replacement, but I couldn’t wait for a week. AutoZone didn’t carry it at all. But O’Reilly was able to get the part to a store by noon the next day, and I had the car working by evening.

Another for rockauto. Don’t forget to look for discount codes online, usually 10% codes are all over the place from forums.

There is a certain podcast I listen to, which advertises a certain place for car parts over and over again. And I am sad to say that the answer popped into my head immediately, from the repetitive exposure to the advertisement.

Suffice it to say it is already listed in this thread.

RockAuto, AutoZone, or a specialty store for the hard-to-find parts

Did Amazon, eBay, junkyards, but now I’m looking at RockAuto.

For Canadians: partsavatar.ca

I think its kind of the Canadian equivalent to rockauto