Konservatorium (
http://www.konsi.ch/Konsi.3.0.html)
-check inside rooms, some rooms have pianos. Best time to go is early in the morning (opening time) 8:00-9:00 or 20:00-21:00 (closing time), when there are usually no lessons. During the day, all rooms are occupied. Check the timetable on the side of the door to see if the room is reserved at a particular time or not.
Irchel Studienbibliothek (
http://www.hbz.uzh.ch/)
-you can ask librarian to reserve piano located in a room right next to library.
I used these free possibilities before I decided to buy one of Roland's KR-series e-pianos. You can purchase e-piano at Jecklin (Kunsthaus)
or Musikhug (Rathaus). Probably you can rent an e-piano there as well..