Welcome to the forum. Love the '64 Custom 880, actually the car that got me into Mopars 35 years ago.
This is going to be a very difficult one given the small volume of the Custom 880s built, so nobody makes them due to low volume to start with.
Now, with that, the 880 was a melding of a couple cars to have something to fit the size it fits back in the day.There is a possibility, abiet rare, that the interior panels are not 880 specific, but possibly another B or C body from the previous year, same year, or (a rarity) a design used on another model the next year (1965).
With that all said, go to the cars section and read a little history for the Custom 880, which I know talks about the two models the car is made from. With that, look at the interior and compare the two models to see if there are parts/door panel pattern, used on one of these two models and see if it may actually be more common and you can replace it that way.
Other thing is, you can then get good pictures and panels are not that difficult to rebuild/recover (etc.) at a good upholstery shop, the backboard itself may be the easiest part to get, the pattern from a picture if you don't have it, and the armrest should be available by matching it up in length and appearance as a common part.
You did forget to mention if it was a 2door of 4door model you needed.