You don't have to have the exact part number anyways. There's a parts counter at the dealership somewhere, at my local one it's in a hallway towards the back next to the shop.
Just talk to whoever is working, and they can pull it up on the computer what you need.
Just take the part number you think you need, ask him/her to pull it up, and then verify if it's what you need and tell him/her you know it's not 10mm if theirs says the same thing. They can help you out, that's what they get paid to do there.