http://www.reeftopia.com/reefaquariumsnails.htm is where I used to always order from. You can get lots (100 or more snails) really inexpensive. Seems like I heard some bad experiences with them the last I checked though. Curious to see others responses as well.
FWIW got my order from reeftopia.com this past week and everything was as advertised. Got 3 free emeralds, free shipping and a free serpent start with a $140 order. The only problem I had was with a few scarlet reef hermits dying and they sent me an Email this evening that they would refund the money for those. Couldn't have been happier.