Only one really big time player and with a proven track record too.
http://www.ensignbus.co.uk/

Try the above link and work from there.
Bear in mind though that Routemasters are selling strongly as fast as they
are withdrawn.
Presently there are quite a few examples on EBay but they appear to be less
than wholesome and are being sold on rapidly in the hope of a quick profit.
The prices appear to vary widely depending on the Engine/Transmission
combination and Body Condition with examples from Metroline being widely
regarded as "Avoid at all Costs" due to the appalling state which they were
apparently allowed to fall into in the run up to withdrawal.
Generally vehicles from Stagecoach,First London and Arriva appear to have
benefited from good quality maintenance and attention and will thus command
a premium.
I would reckon on a spread of between £10,000-£15,000 plus tyres for a ready
to work RM/L
If anybody doubts the sheer engineering quality of the Routemaster design
then take a trip to Edinburgh and have a shufti around the newly
re-refurbished Models which Mac Tours are placing into service on its City
Sightseeing fleet.
The MD of Mac Tours recently remarked that their "New" Routemasters were
perfectly capable of delivering ANOTHER 20 years of revenue earning
service....Now thats what ye call Canny.....!!