Open geo data can be found in a lot of places. Open city data is a great source of geo data in many jurisdictions. Searching for "open data <cityname>" can yield a lot of results. For example, https://datasf.org/opendata/ is San Francisco's open data portal. Some jurisdictions will have dedicated GIS portals.
You'll often find geo data in a few formats:
- A CSV of geo points
- Shapefiles for geo points
- Shapefiles for geo shapes
- WKT (Well-known text)
- GeoJSON