Public toilets?

Any tips for where to find public toilets that are easy to access and relatively clean? I have Crohn's disease and sometimes I will have to find a washroom to use very quickly (like in under 3 minutes). I really don't want to shit myself in Prague.

In my home city public toilets are abundant, restaurants will usually just let you use the washroom without buying anything, and they generally always have toilet paper and aren't absolutely disgusting. Fast food places you generally just don't even have to ask, you just walk in and use it, some places will have a key to give you if they have problems with drug users. Public buildings like Universities and foodcourts/malls and libraries are a good place too.

What should I look for out in Prague if I feel that I need to urgently take a shit?