Download the appropriate technical manual from Shimano, available here. You want to download EV-FH-6800-3605 for the rear hub (FH = free hub), and EV-HB-6800-3606 for the front hub (HB = front hub). They are just a single sheet each. Print it out, highlight the part number you need at the bottom and take it to your LBS. If they sell Shimano, they will be able to order the specific part number you need.

p.s. I did this when I re-built a D/A cassette to have the specific sprockets I wanted. Had to pay in advance and pay for shipping, but it was worth it to get exactly what I wanted.