The Inventory data presented in the Polis Inventory app comes directly from the website polis.stanford.edu, managed by the Stanford University Classics Department.
After the Inventory was published, other scholars, notably Josiah Ober and his colleagues and students, began to capture and grow digital versions (spreadsheets) of the printed material. The result of this effort is presented on their website and their data is made freely available to the public for academic uses and general interest and study.
Further detail about the data and more detailed accreditations than found on this website or in the Polis Inventory app itself may be found on this Standford website.
Keeping with this spirit of data accessibility, users of the Polis Inventory app may download the data from the app in a geojson format from the About page.