does the nasa have one?
or it might be bright on both side.
Two problems with the NASA images though..
a) The Earth images that exist there are very recognizably Earth.. good if you're doing Earth, not so good if you're doing a generic M type planet.
b) The Big Blue Marble data, which is probably the best source for modellers, I don't think has a day and night combined image.. they have a day (lit) map of the entire surface, and a night map of the same. I know because I used it in the past.
There are ways to do up a nice map.. I'll see what I can do up for a tutorial.