Charms are located in many of the gift stores at Disney resorts and at the theme parks
My personal favorite spot to find the best selection of charms is the shop located at the Magic Kingdom beside the Castle called Castle Couture.
Charm bracelets make a great souvenir and it is fun to add on to the bracelets each year. There are charms for each Disney princess, Minnie and Mickey Mouse, mouse ears, flowers, etc.
Each year I let my girls choose a few charms to add to their bracelets. This has become such a fun tradition. You can also find charms at the Disney Store at Downtown Disney.