The o ring you need actually goes on the main drain plug. It is half way up the middle of the main drain plug bolt. You can not see it till you remove it. I think I paid all of 5 dollars for all of these at my dealer and he had them in stock and knew exactly what I needed.
Try talking to the people who actually do the service, not just the parts guy. Maybe they can help the parts guy get his act together.
Here are the part numbers you need.
941009-14000 Washer drain plug 14mm You need 2
T3550123 Washer sealing 24.0x16.3x2.0 You need 1
T3600104 O-Ring, ID 12.1x1.6 You need 1