Hello REK -
I have yet to get a cover for my camper but the best suggestion I have heard re: custom covers for Sprinters is
http://www.calcarcover.com/.
On their website, you should:
1. Roll your mouse over "Custom Car & Truck Covers"
2. Select "Custom Dimensions Sheet" off the resulting pop-up menu
3. Choose "DIMS 228 - Class "B" Motorhome" from the list on the page that comes up
4. Print out and fill in the pdf worksheet that loads up.
It sounds like this company has a good reputation - their website certainly seems to cover all the bases.