Wow! Going to a store to buy car parts, so old timey! I use this service called "Amazon" and sometimes one called "eBay". I amaze my grandpa by finding the exact same part online that he found at Advance/AZ/Napa for half the price.
Example: I just bought a radiator for his '99 CR-V. NAPA had the exact one (Denso) for $113. I got it (same part #) on Amazon for $60 shipped.
No really though, I find the part number of what I want then shop around for who has it the cheapest. Sometimes Advance will be cheapest with an online coupon. Sometimes shipping makes it cheaper to buy in a store. Sometimes I need the part right now and can't wait.
I've had way too many less-than-pleasant run-ins with parts guys, including at the dealer. It's not their fault they don't know every single part in their inventory and every single vehicle it'll fit. And they can't/don't want to be held responsible when they sell you an automotive part that
might fit your tractor/mower/snow mobile/etc... I've worked in retail and I can tell you, there is NO way to know every single piece of inventory you have.
One little anecdote about an AZ employee: I was behind a customer who was wanting to purchase oil. She asked, "ain't I s'posed to run differn't oil in the winter?" Counter guy, "yeah. Your car probably calls for a 5w-30. You can run a 10w-40 or 50 in the winter since the higher numbers protect more". My wife (who is subjected to car talk all the time) was with me and whispered, "that's not quite right is it?". I just hung my head and pretended not to hear them.