For you (I believe you speak good/reasonable spanish, don't you) I woud recommend Segundamano.es "shared flats" page, better printed version when you are here.

I think the trouble with perusing classifieds etc is that you end up making a lot of telephone calls as in a lot of cases the room has already been let. Even worse you can be led in by time wasters who already have a list of 15 hopefuls looking at the same room, but just want to lord it over you in an interview. This happened to me before as I previously stated - it really annoyed me that the promised call to tell me if I'd got the room or not didn't materialise. Consequently I spent another 4 nights paying hostal charges.
This is why, if I need to look again I will use MadridInsider or welcome Spain services. I understand from his previous post that MadridInsider doesn't charge the room-seeker for this, but even if he does it will save time and money (hostal fees, eating out all the time, telephone calls, wasted visits etc) using his services.