Try looking at the first few pages of the Marans thread in the Breeds/Genetics section - It has some Cuckoo Marans. They're the most common variation to find, as they're the only one sold in hatcheries until recently, and with that, tend to have very light eggs. Of course, if gotten from a good, reputable breeder, they can have nice dark eggs though.
I myself just have Black Coppers and Blue Wheatens though.