where to purchase oem car cover for sedan?

Mercedes does not make car covers, just like they don't make T-shirts, golf clubs and other stuff you see at parts department.
Those are items "sponsored" by MB.
So you can go to a dealer and $$pend money on cover with MB logo printed on it, or find the actuall manufacturer and buy the cover directly for much less.
That said, be caution with using covers. They work well on garaged car that you want to protect from dust accumulation, but can wear the paint when used in windy areas.

If used outdoors, Ive heard of several people changing them out every year because the insides/fabric touching the car wears out due to nature and the elements.

but just buy them straight from the manufacturer, don't pay markup at a dealer. I'm sure you could just call the dealer and ask who manufactures them