Gonna be virtually impossible to get one free (I know, I tried). The state film commission in your state can point you to closed down hospitals, but you'll have to pay to film there, have insurance, pay a security guard (likely required by the owner), and get a ginny truck because there won't be any power. If you find someplace CHEAP, I mean the owner really cuts you a deal, figure $1000 a day.
The other alternative is as Indie mentioned, a vet office, dentist office, etc... where you know someone who works there, or can otherwise get a deal to shoot off hours.
The best option (and what I did) is an industrial type building that you can dress to look like a hospital, still probably $200 a day or so.