Places like Rock Auto are probably killing the brick and motar shops in hard part sales to non-professional customers.
I always buy the parts for my fleet of cars from rock auto if I am not looking for OEM and something weird. Point in case, I needed a trans mount for my SHO, I broke it at the drag strip a couple weeks ago. Autozone and checker wanted ~180 bucks for the mount in their "house" brands. I bought the mount off rock auto was a known-to-me brand (anchor) and it was 77 dollars plus shipping. Got it in two days. I replaced the trans mount, and the three engine mounts for less than the price of one mount from a parts store - and it was all equal/better quality.
When I worked at the shops, we never used Pep Boys for parts, as their service always sucked. The delivery time and availability was always way worse than Napa and O'Reilly's.
When I needed tires (245/45/R20) I wanted Nitto 555s. I called around to all the usual tire shops. Bought the tires off Amazon, free 2 day shipping, and took my wheels / tires to Discount Tire and paid them to mount/balance. Came out $80 bucks ahead instead of buying the tires from the tire shop.