where do you find parts?

Depends...

If you're looking to do a factory style resto, then you'll need to see the usual upholstrery vendors (legendary, just dashes, ect.)

On my '65 Valiant, I just wanted something that looked period-correct (something that coulda been original, not 80s velour buckets or something) so I ordered the custom fit vinyl upholstery set from JC Whitney. It's an actual upholstery set that you atatch with hog rings, not the tie-on seat cover type. I got some foam rubber locally and did it myself. That was about 3 years ago and they've held up well. It ain't gonna win best interior, but for less than 300 bucks I'm satisfied. JCW also sells matching vinyl if you need to do the door panels.

I havent tried to make new door panels, but since our cars use flat panels it shouldn't be very difficult. You can get similar hard-board to what Chrysler used in 4x8s. Just be sure to put sheet plastic behind 'em like the originals had -keeps moisture inside the doors from warping the panels.

Good Luck!