Hosz Faszination in Germany offers slotted fret boards in all dimensions you want. Don't look for a bargain there, but it's quality for sure.
http://www.holz-faszination.de/home/node/7 The site is in German but when you send him a mail with your wishes in English, I'm sure he will reply. The mail address is carpenter@onlinehome.de
Good luck
Wim