Once you have decided that you want a pure bred Maine Coon you absolutely have to find a quality breeder to insure that you get a healthy cat. There are a few things that you can do to evaluate the integrity of a breeder.
Almost every reputable and responsible breeder will belong to the CFA. They will also likely have other credentials that include memberships to organizations that focus on this breed. You'll want to have a list of questions ready before you contact the breeder.
What To Ask
Of course you want to make sure that the cattery is well-maintained so asking to see pictures (if you are not local) and visiting if you are local will speak volumes about the breeder beyond any credentials that they can produce.
Ask the breeder specific questions about the parents' lineage and health. Ask if the parents have had genetic testing and if the results are available. Ask about their replacement guarantee for kittens that are found to have faulty genetics AFTER the parents are found to be negative for the genetic anomaly.
You want to discuss with the breeder if they show their cats as well as the heritage of the cats. It is also a good idea to ask how many kittens they sell each year and how many cats that they have overall. You do not want to purchase a kitten from a breeder that has a very high number of kittens being sold each year.
Ask if you can meet the parents. Ask if the kittens visit the vet before they are sent to their new homes. A lot of breeders vaccinate on their own (which is fine) but every kitten should be checked out by a vet before they are sold.
Check out the breeder online. Ask other people that have Coon cats if they have heard of the breeder. Reputation plays a big role in the cat world. Don't be afraid to ask the breeders if they have any references.
A responsible breeder will be more than glad to answer your questions as it will show that you are a responsible cat owner. If the breeder is guarded or does not answer your questions to your satisfaction, look for another breeder.
One last very important point of advice: Get everything in writing! It is imperative that you get everything that is promised or discussed in writing.