All kinds of different things John. The interiors came in the GGD cars. In cars without, the most common source has been a big bag of Walthers interiors I got from a friend. I have acquired seats, sofas and such from estate collections that I've picked up over the years, the old Walthers wood or pot metal parts from the old days, and I have stuff from Precision Scale, the brass folks. In addition to the models, they also do detail parts and passenger car interior parts. There is also Red Cap lines. They are gone now, but their stuff turns up at swap meets or e-Bay sometimes. The model passenger car market has a long way to go until it catches up with the freight car and locomotive markets. It's happening, but it's slow going.
Regarding shorter cars, you are wise! I see many modelers struggling with 85 ft cars and 18" radii, and large six axle diesels or large steam locomotives on 18-22" radii. Now if that's what they enjoy, far be it from me to judge. It's their railroad! To me, equipment hanging over the rails all the way to Nebraska looks ridiculous. I have the good fortune to belong to a club with 48" minimum radius, so for some of my work here, don't try this at home if you're one of those guys!

if I were doing 22-24" radii, I would be running 60 ft coaches (see the 60 ft cars a few pages up this thread) and four axle diesels or 4-6-0's, 4-6-2's, and 2-8-0's. There are also a few smaller Mikes that work on tighter curves and look OK. I once told my wife that if we moved away from the club, I'd probably have to have to liquidate a fair portion of the collection as I wouldn't have any place to run it! The Walthers Proto series car boxes state that a 24" minimum radius us recommended. With brass, you get to find out the hard way, but for most 6 axle equipment, not less than 30" minimum radius is required. 36" is more prudent.