Maybe also just look out for used MTBs with wheels in good shape or just used wheels? With disc wheels, you don't have to worry about brake surfaces being worn down, so it's just a question of still being true and having a hub in decent condition. And there were a lot of disc brake bikes sold 10-15 yrs ago with narrow rims, many of which probably didn't get heavily used. My 2008 Rockhopper disc has Alex rims that are 17mm.