Make it yourself or get someone to custom make it for you.
It's not exactly something normal go players would buy so I don't think they would sell such commercially.
Alternatively, perhaps you might fly to Tibet and look around for it in some antique shops there.
Or perhaps you might take a cheap 19x19 board and paint over the lines of outer edge until you are left with a 17x17 board.
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http://tchan001.wordpress.com A blog on Asian go books, go sightings, and interesting tidbits
Go is such a beautiful game.