Where do you find interior parts?

Chris,
Legendary is generally accepted as having the best quality upholstery. You can buy directly from them but I buy most of my stuff through Dante's Parts. You can usually get a price as good as buying directly from the manufacturer and he generally has great service. Plus you can buy everything you need from one source. I've also seen him get parts delivered faster than buying directly.
I have a '73 Dart Sport and I bought the dash pad, carpet, console carpet, package shelf, door panels, seat covers, hinge covers, trunk mat, and spare tire board, all from Dante.
There are some sellers on Ebay who sell cheaper seat covers but I haven't heard anyone brag on them.
Hope this helps,
Dallas