For the past few films I've been involved with we've used libraries as well. They often have private function/meeting rooms, and even, in our case, gave us permission to go in there in the evenings and on weekends when the library was closed. And it was free.
Having said that though, several times I've had actors come around to a house to audition, and whilst perhaps not appearing as 'professional', some of the best performances were given this way. I think the actors tended to feel very relaxed just 'chilling' on the couch talking over a cup of tea, and this comfortableness seemed to help take some of the nervousness out of their audtions and several of our actors have been cast this way.