There is an app for that. When you really Gotta Go,
Flush Toilet Finder is an app for smartphones that helps you find
free public restrooms.
The app on Android and iOS is handy if you are in a strange city
and you do not know where you can go or if you are shopping, and
emergency hits.
Open the app and it searches the area around you. Its database
covers over 100,000 public restrooms around the world and the
database is constantly expanding. When you find the one you want,
you can get directions via Google or Apple Maps. Flush also lists
if a restroom has a fee, requires a key, or if it provides
disabled access. LINK