Products we love!
(this list is by popular demand, and will be updated regularly) (I make no money from these links!)
Scratching Post - It's tall and doesn't shred all over the floor! Need I say more? Maine Coons are looooong. Normal posts don't cut it.
Sheepskin pelt - Oh man do cats love these! I have two just in our living room! Don't get the fake kind - cats can tell the difference and adore the smell of real wool.
Litter - so far this is the only one I've found that clumps well, controls oder, and is fragrance free. Please contact me if your kitten is eating litter - regular clumping litter can be fatal if ingested! I have other suggestions. 
Litter scoopers! Not glamorous, but it's a big part of our lives! My husband's favorite here, and my fave here. 
"H" Halter - There are tons of colors, and two sizes. The small size will last a few months at their new home. Anything in the "H" style I like. 
Harness for grown MC - The 12"-20" should fit your big baby! 
Nail Clippers - these are a strong, sturdy option, for the adult MC I recommend the size Medium (3mm). I don't like they way they feel in my hand, but they are very sharp and able to clip nails without splitting them, which I appreciate. 
Ball roller - Their ongoing love of this toy continues to amaze me. Mostly for kittens. I've seen them cheaper at TJMaxx before.
Probiotic - I trust this brand a lot. Our homes are too sanitary! Cats (and humans) need more bacteria to thrive. Bunny poops are ideal, but in the absence of a bunny poop donor, probiotics are helpful.
Furwood Forest Cat trees - Expensive but amazing. They're large, can fit even the largest male Maine Coon, and so sturdy that my 4 year old daughter climbs it as well and it doesn't sway! Someday I hope to build my own, but until then, these are outstanding, and aside from my beautiful cats (and charming daughters), these cat trees receive the most compliments at my house.
We have a PetLand store locally, and they sell very sturdy cat trees, which we have one of. They also sell puppies, and I don't like supporting the resale of purebred animals (so much is lost and risked when you don't buy directly from the breeder!). So I'm torn on recommending them. 
Cat Tree King - These are huge and will fit even the biggest male Maine Coon! They seem to often have sales going on, so keep an eye on the website. We have one in our rec room/basement, and the cats love it. 
Foliage tree houses - I don't have one here, but some of our pet homes do, and love them. They're beautiful! And so unique.
Cat crate - For the adults heading to the vet, I like our hard crate that is approximately 24"x16"x15". We don't have the one I linked there, but it seems like a reasonable size (the larger one), and is cheap. There are zillions of options in different sizes, and if you're going on a long road trip, of course you will want something bigger.
Scratching rug - Some of our cats seem to really like sharpening their claws on the ground level - meaning, a rug. I have some jute and sisal rugs here at home for those that do. Ikea carries them, but I'm certain you could find more at TJMaxx or elsewhere! Just make sure it's nice and stiff with a good solid border so can't easily get bunched up by your big strong cats! (Yes, that includes the girls.) Here's another option. When purchasing, I recommend a "feel test" - run your hand along it and if it feels nice and bristly and stiff (like tree bark!) then they'll probably like it!
Cord covers - there are lots of options, but this is one we've gotten good feedback about. 

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Early spay/neuter CFA study
Why keep your cat indoors?  Article
This is a good article, though it does leave out some key reasons, including kidnapping and unwanted pregnancy if your pet isn't neutered.
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- The Surprising Economics of a Purebred Puppy
- So You Want a Litter?
- Why are Purebred Cats so Expensive? 

Other MC Breeders I know personally and trust:
iCoons Cats
Br'er Coon - she recently retired  =(
​Bloomingtree Maine Coons in Germany
Audra Kuliese - Su Meile in Lithuania 

Interesting reads:
Over-vaccinating? - one of the most important things to read!
How Big Is Your Maine Coon?
America's First Show Cat - CFA.org
Main Coon Breeders and Fanciers Association
​Maine Coon colors (not an exhaustive list)
The Origins of the Maine Coon - pawpeds.com
Maine Coon Training - catchannel.com
Do People with Pets Live Longer? 
9 Reasons to get two kittens instead of one
​My sister's Etsy listing for hand spun Alpaca and Angora wool from her little farm (she's so cool!)