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.
Yes, it is the 4 door sedan...I suppose I could try to find a Polara or a Newport panel, but I have a feeling that it would be almost as hard to find those as it would be an 880. The interior is pretty much your standard ribbed leather, same as the bench, but the previous owner customized the other panels with (of all things) faux black and white snakeskin, which won't be too hard to find....I just wanted to try and keep it as original as possible by maybe catching lightning in a bottle and taking a stab that someone might have a panel out there.
You might accidently find one, but if the back panels and armrest are in good shape, recovering them is your best option. Since the armrests are shrink covered (if they have been changed, don't know), but Just Dashes can recover them as original, they have all the patterns and your best choice to get them looking new (along with foam that lasts longer than original to boot).