Adopting a Maine Coon cat is a great idea. There are cats in shelters across the US and around the world that can benefit from having a loving home, but you should be aware that adopting a pure bred Maine Coon cat from a shelter may be more difficult than you think.
Typically, shelter animals are dropped off or picked up off the street. They are abandoned animals with very little history known about them. This means that it is a crap shoot as to whether you will actually get a pure bred cat.
Of course if pedigree is not your priority, you may find a cat that is very similar to a Maine Coon. You may be able to find a cat that looks like a Maine Coon in a shelter, but understand that there is no guarantee or a very high likelihood that such a cat will be pure bred. This is not to discourage adoption or rescue, but to make it clear that with almost any rescued cat tracing the lineage is very difficult if not impossible.
Purchasing a Maine Coon Cat
Purchasing a Maine Coon cat from a breeder is going to be expensive but it is also a way to ensure that you are getting a pure bred Maine Coon cat that is healthy. You can expect to spend anywhere between $400 -$1000, which does not include breeding rights.
Adoption is less expensive, but there are still some fees that are usually associated with adoption that can add up to close to $200 and for that fee you are not guaranteed that you are getting a Maine Coon cat.
Typically when you purchase a cat from a breeder you are also going to get a history of the cats lineage, and you are going to get a kitten that is healthy and free of genetic issues. Guaranteeing the health of the kitten is typically not possible when you adopt a kitten from the shelter or rescue. Most breeders do in fact guarantee health up to a certain point.
Ultimately it is a personal preference that should direct your decision. If you want to do the most good then adoption is clearly the choice. If having a pure bred is very important to you, then purchasing one from a responsible breeder is the way to go.