My local had the greek store by our school order them for us. They were $25 each, same as a nat'l, and there was no minimum.
Joetoga.com also sells custom silver lavaliers for $8, with a minimum order of 15, and then no minimum after that. even if you aren't going to need 15 right away, its still affordable for the chapter to order the extras, and have them on hand for future sisters to buy. Or for no minimums (if its you ordering one for yourself) you can get a silver lavalier for $20 or gold for $34.
Joetoga.com products for locals