The best location is one the visitor sees upon entry to the venue and/or exit out of the venue, The shop should also be accessible to the visitor without buying a ticket to the institution. Some shops generate significant volume from visitors just wanting to shop, especially around holiday time. Additionally, if the shop can have outside access, it can be open even when the institution is closed. This is not possible in many facilities. If the shop is not highly visible, be sure to have good directional signage within the building or on the grounds and feature it prominently on the website, all maps, and collateral material.
