I have purchased them from Hallmark and Barrington's.
I've gotten the other brands from the Dollar Tree, Limited 2, Michaels, and from eBay (the post before is the same seller I bought mine from). The eBay seller was great - I think I got 11 or 12 Disney charms for about $1/piece.
Limited 2 had the wrist band bracelets that you can put the charms in too which was cute. I got the bracelet yesterday and the charms there were 3/$6.50.