You can buy them at the Magic Kingdom at the jewelry (engraving) shop and at Downtown Disney at the engraving place in the Christmas store. Those are the two places I know of.
This year they have 2 new badges for the happiest celebration on earth: a gold one with Mickey Mouse and a purple one with the castle and Tinkerbell. You can also get a Princess, Stitch or Buzz Lightyear one.
Enjoy! They are fun keepsakes, aren't they?