Great car - I just responded to your welcome wagon post too.
So the question "Who are best quality part sites" is really broad. Unfortunately, I have found that there are many crappy reproduction parts and quite a few good ones too. I can't offer one universal "good parts" place, but here are a few of my thoughts to get us started:
Seat covers and interior panels: Legendary interior is the absolute number one, and I think for good reason. If you really want good fitting, proper seat covers, they are the place to go to.
Carpet? Honestly not sure - there seem to be many places that sell these. I think Legendary has these too, so maybe you can work out a package price.
For lots of small parts, I have been going to Classic Industries lately. They have some crap too, but most of the general vendors do as well. I don't want to spend your money, but I am a big proponent of replacing the wiring harnesses on these cars. The copper is fine, but the insulation was never intended to last 50 years. Save yourself major hassles (and maybe avoid a fire too) by replacing them. The absolute best are M&H wiring, sold (I believe) exclusively by Year One. Wait for a 30% off sale and snatch the them.
For weather stripping, Metro makes the best stuff, in my opinion.
I am sure others will chime in, but there are a few suggestions to get you started. Now get to work!!!! (smile)