Finding AT cases should be easy if you know where to look, for instance I recently got a NOS case for free from a computer shop when I bought a portable HD: when I was paying, I noticed that there were a couple of cardboard boxes labeled "AT Computer Case", I inquired about it and the guy from the shop told me these were old stock he was trying to get rid of and gifted one to me (he also had a couple of boxed Voodoo 4 and 5 but those were "not for sale" 😢). Not bad, considering I live in a small country with a population of 3.5 million.
However, I agree with you when it comes to finding used cases in reasonably good condition. A week ago, I bought a lot of 10 used AT cases for less than USD 8. Of those, 5 were completely obliterated and the other 5 had noticeable amounts of yellowing, rust and of course lots of dead insects and unidentifiable goo. Still, I gave them a good cleaning and will try to sand and repaint them when I have some free time.
I have never done this kind of restoration but I think it is a better choice than going through the hassle and cost of building new ones, though it would make for an interesting project if you have the means to do it.
By the way, a question for people who have restored old cases, what is the better solution for heavily yellowed plastic front panels? Just painting them over or applying some peroxyde based treatment (like Retr0brite)?