In Chicago there's of course Central Camera, right in the loop, on Wabash Street, not far from the Art Institute. You can go there almost any day, but the atmosphere is friendlier on Saturdays. Their used gear inventory used to be larger, but they still have fairly good offers on medium format. Scattered Leicas and a few Classic Japanese RFs can be found there.
In case you want to purchase, they may seem a bit over the usual but if you consider their one-year warranty deal, it's peace of mind.