If you want them for their dark eggs, which is what they are bred for in Europe, then you maybe disappointed if you purchase them from a hatchery.
You want to purchase them from a breeder in that has breed to egg color and production, good Marans should be almost as good of layers as leghorns.
I am a sneaky SOB so I would first purchase a dozen hatching eggs, to check the color and size. Why? Because American breeders tend to breed to the cosmetic standards not the "purpose", egg production, egg color, egg size is often ignored.
There have been several good articles in the Poultry Press about this over the past few years.