I'm with Steve on this one too. These cars are so unique and so few were made that finding interior parts can be close to impossible. Interior parts are more coach builder and color scheme specific. If you potential car was unique when it was built in any particular way (a rarer cabinet setup, odd interior color, etc) finding the parts that you need may never happen.
In my case, I've owned my my Cascade Green '66 M-M for 11 years now. Though the interior is certainly presentable, it's far from perfect. The original owners were hard on the car and the owner after that had a cat living in it for many years. This all resulted in a front seat that has several dime sized holes from battery acid and scratches on the one side where the cat used the seat as a scratching post, curtains that were in rough shape and that reeked of cat pee, a rear linoleum floor that was completely painted over in avocado green with all of the chrome and aluminum parts in the back compartment painted over in a silvery gunmetal gray paint and a broken glass partition.
Most people would consider all of this a disaster, but I've worked through it. For the floor, the easy fix would have been replacing it. Armstrong no longer makes the linoleum pattern that's in my car though. I searched for it and had no luck. There isn't anything on the market nowadays that's quite like it, so I had to peel the paint off of the entire floor by hand. This was awful, but doable. I tried using tools to speed up the process, but it only scratched the floor, so I had to wait for really hot afternoons and pull the latex paint off by hand, with the help of a hair dryer sometimes too. Now that the paint is gone, the floor looks a lot better, but it's not perfect. There's still wear on the floor, some discoloration in some areas and scratches, but it's better than both of the alternatives - having new, but incorrect flooring or having that horrible avocado painted floor.
Taking the gunmetal paint off of all of the metal pieces in the back is much more annoying and time intensive. So I've just cleaned up the most visible bits and the parts that were peeling. The rest is still there and I just take care of it gradually by removing it from parts when it's flaking off. It's not incredibly noticeable like the paint on the floor was, so it doesn't bother me too much.
As for the curtains, the fabric isn't made anymore either and it's tough to find anything similar. Cascade Green just isn't a popular color and it hasn't been for decades. Even living 10 minutes away from the Jo-Ann Fabric headquarters and superstore didn't help much. After searching on and off for fabric for six years, I finally found something close enough. The color is right but texture isn't quite right. I know it's not correct, but a random person doesn't so it's all good. They might not be completely perfect, but it's much better than driving something that smells like a cat litter box on wheels.
The easiest thing to deal with was the broken glass partition. I found a '67 M-M at CW Coach that was going to be scrapped. I went down there the day before it went to the crusher and pulled random parts - including the partition. M-M slightly changed the partitions between '66 and '67 and I know the difference, but it still fit.
And then there is the acid burned and cat scratched front seat. Good luck with that one. After talking to Tom Caserta, the old sales manager for M-M, I know that there were literally THREE Cascade Green M-Ms made in all of 1966. Am I going to be able to find one of the other two? I seriously doubt it. And if I did, is it going to have a perfect front seat in it? I really doubt that too. 1965 had a different design for the seat and 1967 had a different design too. I'd maybe settle for a seat from one of them even if it was the wrong year and wrong pattern, but I can't find a green '65 or '67 M-M so it doesn't matter. Getting a green seat from a different coachbuilder won't work either and it definitely wouldn't match the rest of the interior, so that's not a viable option. Because of all of that, the acid burned cat scratched seat is just something I will always have to deal with.
The outside of my car was a disaster when I got it too. It was five different shades of green, the rear quarter panels were so rusty that they were non-existent, all of the belt line trim was gone and replaced with some horrible stick on boat trim that used to be sold by the roll and the rear bumper was rusted out so bad that you could see through it in some places.
How does the exterior look now? Pretty awesome. The body shop had no problem welding in new metal for the quarter panels and they had no problem getting Cascade Green paint made for my car either. I even had a friend of mine send me a partial set of belt line trim for free. The free trim was from a '67 M-M in Texas (thanks Alex!!), but it didn't matter because it fit anyway. The rest of the trim was off of a '67 S&S that I pulled myself from a car at CW Coach. It all looks great on my car. I even got a new bumper for my car from that '67 S&S at CW Coach too. Sure it's technically from the wrong year and the wrong coachbuilder, but it was very easy to modify into a correct '66 M-M bumper.
Anyway, the moral of the story is that my car will never be perfect on the inside because the parts I need just don't exist. The outside was pretty easy to fix because the parts were used on other cars too.