Amusingly I've owned two neo geos over the years, and found both of them at flea markets. I've also seen neo geo games at game stores and fleas.

Hilarious side note: The first time I ever saw Vectrexes in the wild was in Japan, where I came across not one but three Bandai units in game stores. I've only ever seen one stateside for sale, and that was the one I bought at CGE 07!