It's a standard part as fitted to many older Hondas: part number is
It was used in the Shadow 600, CBR1000F, CBR600F, Goldwing 1500, early Fireblades etc. You can probably get one from a breaker's.
Otherwise if you really need a brand new one you can get one from David Silver: David Silver Spares - VTR1000SPY 2000 Thermostat switch .
How about this?
Fireblade CBR900 1998 fan switch thermostat #4413 on eBay (end time 19-Jun-10 11:44:35 BST)
HONDA VFR750 VFR 750-RC36 RADIATOR FAN SENSOR SWITCH on eBay (end time 09-Jun-10 16:18:28 BST)
Honda VTR1000 F-2 Firestorm 2002 Fan Switch on eBay (end time 16-Jun-10 14:46:59 BST)
All UK based auctions, always the same part.