File a service request, "other" category, and write that you'd like to order parts with the part number(s) you want. They'll draft up an invoice, and if you approve they'll order the parts.
I've heard sometimes they'll even have mobile service drop off parts for free but that hasn't happened for me.
Also they may or may not remember to check and notify when your parts arrive, so if you don't hear from them for a while, send a message to ask if the parts are ready.
You'll have to pick a service time slot but they'll understand that it's meaningless, once your parts arrive you can just go pick them up anytime they're open.