In the US, Cal-Sportbike is the authorised distributor:
http://www.calsportbike.com/cgi-bin/...ex.cgi?pid=800
If you watch Ebay, or bike forums, usually they'll show up for sale.
This is much more reliable than trying to adjust the resistor described in the zeroyon article. I actually got in touch with the author of that article and he said it was a good product.
I haven't had experience with it just yet, but it should work. Should make your speedo much much more accurate and allow you to adjust for gearing and tyre changes. I'll dig more into it and post my finding later on.