Not very close to downtown, but I bought my molecajete at Supermercado Chapala in Rogers Park. It is pretty heavy duty, and I have been very happy with it. It looks like solid volcanic stone, but it might be actually cast from some sort of concrete-like compound. I bought it over a year ago, but I would imagine they still sell them. They also sell both aluminum and wooden tortilla presses.
Supermercado Chapala
7117 N Clark St,
Chicago, IL 60626
(773) 465-3907