Bus Lists On the Web
A huge and excellent resource of lists covering: chassis lists body lists year /registration lists and more. Well recommended!

PSV Circle
The ultimate source of bus related data in the UK. Details membership and publications

RAC Registration Checker
Go to bottom right of the screen. "Obtain your next prepaid check?" Enter registration and will give basic chassis details

The Bus Station
Catalogues all manner of bus related links globally

Woolybus
Fleetlists available to purchase