We found the short term place to stay only today. I can not tell you how happy I am (even though the place is near airport) as I thought we'll end up under the bridge. Either there is no availability or too expensive. Try these, although almost all of them are from 2300 francs a month (for 2 people). Not all of them got back to us, and the ones who did didn't have anything available.

Aparthotel Rigiblick
Emma House
PABS
Sleep in Zurich
Vision
City Appartments
California House
Accademia
Swiss Star
Lodge finder

This will be useful too

Housing links
WhiteBoard
Craiglist