I buy the o-rings at autozone,they do have them.
You can't just go by what one sales person tells you at the auto parts stores because some of them simply don't know as much as you would think they should.
I've asked for certain items before and was told by one salesman that they didn't stock that item or couldn't get it. I then walked over to another salesman without telling him anything about what the other salesman had told me and asked him about the same item and he went in the back of the store and came back with the item in hand.