MizPee is a website that helps you find public bathrooms in your area. This site is in its Beta phase so there aren’t many listings. Of course when you have to go, you won’t have time to find a computer with internet access, so this website is mostly a mobile service. You goto their website and you can navigate to city and it’ll show you a listing of public bathrooms. Again, not very accurate but should be much better once it grows.
For me, it’s still easier to go into a store and ask to use the bathroom. It did give me ideas for a website or two.