the waldorf school in philly sells them, as does jazams in princeton, which is a bit further but a great toy store in a great town! there's also a new store in mt airy, the nesting house, which either has them or will probably get them if you ask.
my 8-year-old still wears hers almost daily, she says it's helping her with her permanent teeth.
spottedfox, i don't know if that would work for your son, as the amber necklaces are not intended to be chewed, rather the analgesic properties of amber are absorbed through the skin in order to provide relief from teething pain. perhaps something more like this would be appropriate for your son, http://smartmomjewelry.com/tween/tween.asp, as it is designed for putting in mouths and specifically targets "chewers". good luck!