War Surplus Market, sometimes called the American Market or "Cho Cu" or "Khu Dan Sinh" in corner of Yersin and Nguyen Cong Tru, District 1. Hidden behind rows of hardware and electric supplies shops, just brace yourself and enter. Dense warrens of stalls with old American military gear of indeterminate authenticity (e.g. "nice collection of so called authentic GI's Zippo lighter from the war era"), cheap t-shirts, and military paraphernalia. Don't hope to find a genuine Marine Zippo; they're all fake now.
