Finding a Kitten
First, put "pet shop" right out of your mind. No reputable breeder, of any kind of cat or dog, would even consider selling an animal to a pet shop. Kittens found in pet shops are from what would be considered "puppy mills" in the dog world. Pet shop prices will always equal or exceed what a reputable breeder would charge. And you cannot evaluate the conditions the kitten was raised in, or meet its parents, or other adults of the breed. It is a massive mistake to think about "rescuing" a puported MC from a pet shop.
Almost as bad are the backyard breeders (abbreviated BYB). These may be people who truly love their animals, but have started out with pet quality cats (perhaps purchased from a pet shop) that they breed together. They don't register the offspring, show their cats, or maintain any contact with the breeding community at large. They do not necessarily provide the same level of research into health issues, breeding their cats to the accepted Standards established for this breed, or socializing their kittens. They have no intention of improving the breed, and they have no network of other breeders to contact to help resolve a problem. Their pricing may be the same as that of reputable breeders - but people end up paying for a cat with questionable heritage, that may or may not resemble a purebred MC, with no knowledge of what health issues may be lurking in the ancestry of the cat. See following section on "Questions to Ask the Breeder" and "Visiting a Cattery" for some ways to make sure you are dealing with a reputable breeder.
The very best place to find a MC kitten is by attending a cat show in your area. Nope, the kittens themselves may not be there, but the most important thing is finding a breeder that you trust and want to work with. You can find out about cat shows in your area by reviewing the show schedule in Cat Fancy magazine, or by calling the Cat Fancier's Association (CFA), who can refer you to shows and breeders in your area. The phone number is (732) 528-9797.
Cats are also advertised in various magazines - Cat Fancy, Cats USA, Cats, and in the local newspapers. Many legitimate breeders advertise in all of these publications, but there may also be BYB's - so you will need to ask plenty of questions first.
Another referral source is the Maine Coon Breeder's and Fancier's Association, or MCBFA. Contact Deborah Hall, Corresponding Secretary, at 4405 Karrol SW, Albuquerque NM 87121. They will not recommend specific breeders, but will send you a list of breeders in the area that belong to MCBFA. That doesn't mean that only MCBFA breeders are "reputable", but most are reliable. Again, you need to ask your questions.