There is no definitive answer for this question. There are quite a few breeders who sell nice Marans with good dark eggs. There are also rather a few sharks. With Marans it can be very much buyer beware. Those breeders with the best reputations probably have the longest waiting lists.
You need to find a breeder that has the birds you like and chat with them a while and find out if the eggs are what you would want also. Dark eggs can be misleading with a photo so a visit before purchasing is a wise choice but sometimes not easy.
I happened upon a gorgeous pair of wheaten Marans and then the hen laid me some gorgeous #7 eggs. I got them at a steal of a price. You can bet I am overly fond of this pair and have raised a good many chicks from them.