The light bulbs used for saloon and blind box lights on the RLHs had a
base called "B22D" (stands for British standard, 22mm cap diameter,
Double pole). They used "pearl" (=frosted glass) 24 Volt bulbs rated
at 12W (although 20W are sometimes used for brighter saloon lights).

I believe they originally had smaller bulbs on the offside (to give
better head clearance with the sunken gangway) and larger diameter
bulbs on the nearside. Today, I don't know of any suppliers of the
large diameter bulbs anymore. Similar were used on Routemasters,
although higher wattage. The smaller diameter ones are still made
however; approx 38mm wide by 53mm to 60mm high from top to bottom.

These are available as model numbers LLB816 (for 12W) and LLB821 (for
20W) in the 2006 Lucas catalogue, downloaded from
www.lucaselectrical.co.uk complete with drawings of all fittings.
Other vendors commonly keep the code numbers but put their own
prefixes in front, eg ABC816. Suppliers in the UK recently included
www.ceag.co.uk and www.elta.co.uk and prices are around 0.50 GBP each.
There do seem to be some web sites charging much higher premiums, 2.50
GBP or more!

I'm not sure what headlights you have, but they may be "P36D" base
(not the same as B22D) 44W, which are code number "330" (transverse
filament) or "331" (axial filament). These were non focussed single
filament bulbs. All our RLHs now have replacement, dipping headlights
so I can't compare them.

BA15D base bulbs can also be found in the Lucas catalogue.