Here's a few I dug up with
...
http://www.ncs-systems.com/
http://spares.jicconline.co.uk/
http://www.protech-uk.co.uk/speedo_con_delimit.htm
http://www.revtech.co.uk/
If you need a US supplier, check your local phone book for places which recalibrate digital speedometers. I expect they would be able to source the chip.
Some companies sell chips which can be programmed to adjust the frequency in order to compensate for changing the size of the tyres.
Phil.