I have a magnetic white board in my room so magnets are a necessity. One way to have useful and custom ones are to quickly make my own.
Stickers with a thick backside (or cardstock to stick the stickers to)
Magnets (I prefer the heavy duty round ones at Michaels Craft Store)
I bought thick stickers that match the colors of my room at Jo Ann Fabrics on clearance.
Peeled them off the packaging WITHOUT peeling off paper that exposes the sticker on the back.
Bought the strong round magnets at Michaels craft store and hot glued the magnet to the back to the sticker.
Now I have custom magnets for the classroom.
If you would like more organizing tips check out our book The Uncluttered Teacher.