Rectangular magnets are available in Ann's CafePress shop.
There are a few circular magnets, but most images don't fit that format.
Magnets have a metal shell with a wrapped edge and a plastic covering.

Sample Magnet

Visit the shop and click on the "Browse by Product" dropdown to search for magnets.

* * NEW * *
Ann has just found a source for square magnets - they're similar to the rectangular CafePress magnet shown above. Visit her Zazzle Shop to find high quality magnets featuring her square format art! She's just opened the shop and is in the process of adding art. If there's a square image you'd like, but it's not there yet, please contact her to make a request.

Click on any image to see the magnet on Zazzle