So, I have 1964 Dodge Custom 880 Convertible. I really want to find car seat covers that spend a ton of money on custom upholstery. Since my "64 880 was derived from other models. What other model car seat covers could i use instead? I have a split bench seat in front, 40/60 split.

possibly chrysler 300 non letter series, 62-64.

Either other C-bodies or a generic aftermarket cover.
If you want an OEM look, try SMS in Oregon.
12 years ago I found NOS covers for a '64 Savoy wagon at a place in MI while searching for material.
Can't recall the name exactly, something like Auto Trim.

pics of the car?

I drove the 4dr version in the 90's.

it sort of grew on me.

Try googling
bench seat covers for classic cars or Made to fit seat covers for older cars JC whitney used to do them years ago, don't know if they still do