Personally i'd wait until they get a V3 update before deciding to buy them (Although there hasn't been any news suggesting they'll be out anytime soon :S)
Either way you can either buy them on Crypton's site but I don't think they ship to the West so your best bet is eBay, or as suggested above wait for their V3 update so they'll be up on the Vocaloid store.
http://www.vocaloidotaku.net/index.php?/topic/19636-where-to-buy-vocaloid/
This is a useful topic at VO for buying vocaloids.
Hope you know Japanese though, if not, wait for their English bank (this is going to be a while though). As Serza said too, their due a V3 update.
Personnelly, there are better vocaloids out there then the Kagamines; I would consider them. Seriously. But if your dead set on getting them... :-/