One website is Freecycle.org where you can sign up for automated alerts for your area (worldwide) where "offers" are posted on a first come first served basis. Some great finds are to be had, especially for the classroom, crafts, and outdoor resources.
Another great way to scout for free items is by asking local businesses for unwanted items in advance. This includes promotional shop items like posters and cardboard cutouts (great for your classroom library) and items for science, math, and crafts! Think grocers for cardboard, DIY shops for unwanted scraps, and design stores for wallpaper and fabric remnants.
Good luck and just remember, the answers always a "no" unless you ask. :)