http://www.seiyajapan.com
http://www.higuchi-inc.com/index-e.html
http://www.creationwatches.com
http://www.shoppinginjapan.net
Personally i'd go with Seiya. I have paid VAT/Customs on every order i've made, make sure you budget accordingly. 20% plus handling charge (usually £13 with Parcelforce)
For example, a MM300 would you cost you $1800 (about £1250) but then there will be an additional VAT charge of around £250