I am not able to direct you to any source where to buy a power rear sunshade, but I van confirm that it is very well possible.
I have added an original power rear sunshade in my W202 C class.
You will need a complete sunshade mechanism, a different rear deck (with the slotted hole for the sunshade to stick through), the wire harness (very simple) and the switch. You can file a hole for the switch in the center console panel yourself. I have been mailing with someone who wanted to add a power rear sunshade in his C class as well, and he mentioned that it would cost him approx. US$800,- when purchasing all parts new from the MB dealer.
with a minor 600+k