Save Those Freebie Magnets...
You know the ones... you get them at your doctor's office, the vet's office, the insurance agent's office, home shows, etc. They are usually business card size, but sometimes much larger.
If they have a "plasticky" front, it will easily peel off, leaving a layer of stickum. You can use the whole magnet with some of your own computer-generated art (business card, little motto cards, etc.) or cut into strips for smaller stuff. Peel one edge loose before cutting into strips... makes it easier to get the plastic off.
If the front is paper instead of that plastic, it will still peel off, but you'll have to soak/scrape off the paper residue, then use some glue to attach your creation.
Now I know you can just go to the office supply/craft store and buy magnetic sheets and strips, but free is better, especially if you don't have to make a special trip to the store. And often you don't need a whole package of the stuff, just a little bit.