Where can you find Polaroid 900 film?

Unfortunately, you can't. The Polaroid 900, manufactured from 1960-1963, used 40-series roll film that was discontinued in 1992. Some people have had success converting the camera for use with 100 Pack film (instant exposure) or standard 120 roll film, but claim results are only acceptable. The conversion is permanent and destroys any collectible value the camera may have.