Iron filings can usually be purchased or ordered at your local teacher supply store. You can make a bottle last for a long time with a little thrift. Teach the students to keep a file folder under the filings at all times. When they are finished playing they can use the folder to pour the filings back into the bottle. If a student raises the folder with filings on it then they can put the magnet underneath to see patterns of magnetic force surrounding the magnets and passing through the paper.
Iron filings are a good material to have handy in almost any magnet project.