In general, Black Coppers will give you the best egg color. That isn't aways so, but in general.
So good breeders;
http://www.bevsmarans.com
http://home.earthlink.net/~kdoctor1/
Join the Marans Club of America and dark egglayers yahoo groups for starters.
I have chickens from several different breeders. The questions to ask are
"How dark are your eggs on the French egg chart". Look for at least a 4, preferably a 5 or 6. If they say they have 8-9s they are probably lying through their teeth.
"What lines are they from?" Hatchery birds are gentle, nice, not dark layers. I can tell you that from experience as well. Most are just darker than a RIR, say a 2 or 3 on the egg sale.
"What size, shape, quantity does the parent flock lay"? You can get good marans that lay large eggs, but if they lay high quanities, the egg is likely to be a lighter egg. From my experience the really dark layers only lay 3-4 eggs/week. It takes that much time to get the dark coating on the egg. Marans eggs should also be nearly round, making it hard to tell point from base. You should be able to tell, but it shouldn't be as easy as say a RIR.
"How big are the birds, and what is their conformation like?" One of the common problems I have seen in marans is their legs can be too close together.
"Feathered shank or Featherless shank?" Makes a difference. French - feathered, English - non feathered. Pick your preference...Most people are shooting for feathered shanks.
You shouldn't have to pay the rediculous prices people are asking for the marans eggs. If you are lucky, you will get 8-9 to hatch out of 12. I have seen them go for $460/dozen. That's $51/chick. Let's say you get 4 hens, eikkkk!!! That's over $100/layer!!!
The last thing I can say, is ask for references. I paid $240 for bad eggs. I hatched 5 out of 29 from one group and 3 out of 26 on another - two different "breeders". A lot of the eggs were dirty, odd shaped, and odd sized. I think some people knowingly send bad eggs out because they are fetching such a good price. When I asked them about it, it was "Too bad so sad. Sucks to be you". I decided to go with a true breeder and I hatched out 9 of 12. Go figure. I learned my lesson there. It pays to get on a good waiting list, and wait...
Edited to add...If you are interested in eggs for next year, get on the breeder's waiting lists now. The waiting lists can be a year long or more...