240v 300w BA15s A1/183 lamp
Please note that these projector lamps are virtually impossible to obtain anywhere in the world. In the early 1970's, several lamp manufacturers were happy to open up a dedicated prduction lines to make theses special lamps in response to multiple projector manufacturers making a range of slide projectors in the 1970's and 1980's that were specifically designed for this lamp type.
With a declining interest in slide film in the late 80's, these projectors were no longer being made. Without a market to utilise these lamps, the lamp manufacturers closed down the production lines that were making those lamps, as to the best of my knowledge, no other light fitting or projector used these types of lamps.
Given that no new lamps have been made anywhere  in the world for about thirty years, and they had limited use other than as a projector lamps, there are very few of these lamps still available for sale anywhere now. I am aware of a  limited number in America, but these are 110 volt lamps, not suitable for 240 volt projectors.  
I have kept this advertisement up in the hope that I may uncover an old estate with one or two good condition unused lamps. But honestly, that situation is becoming less and less likely.
There is no suitable alternatve lamp as they were purpose made for that range of domestic  35mm slide projectors only.  There is no adaptor that would permit the use of another type of lamp. 
The only possible options are to have the projector lamp housing professionally modified to accept a readily available lamp. This would envolve making chages to the lamp mount, socket, optics and ventilation. I have done this to several of the smaller projectors I have here, but the time and parts I used in the modification well exceeds the value of the projector.
If you are looking at purchasing another projector instead, may I suggest the Kodak SAV series. These are an extremely robust, professional type 35mm slide projector with an 80 slide carousel on top. Very good specifications and the huge plus of them is that they use a lamp that is still manufactured, there is no shortage of them worldwide and at about $23 each, are about a third the price of these blue cap lamps.  
I do have some Kodak SAV projectors, and accessories, available for sale on this site under "Slide Projectors"  
The last option is that you rent a projector. Through our rental side, we have 35 mm projectors available for hire, that may be an option if you just wanted one for  a single night family slide viewing event.
I am sorry that I can't readily assist you with a lamp, but trying to source spare parts for anything electrical or electronic pre 1980 is becoming increasingly difficult. Especially so lamps.
Chris Markham   June 2020
  
  
300 watt 240 v BA15s Projector Lamp
|   Cap/Base  |    15mm Ba15s/SCC  |  
|   Cap/Base Type  |    Bayonet  |  
|   Diameter  |    31 mm  |  
|   Equivalent Code  |    7066N  |  
|   Length  |    81 mm  |  
|   Lumens  |    6900 lm  |  
|   Average Lamp Life  |    25 hrs  |  
|   Shape  |    Tubular  |  
|   Manufacturer  |    Bell & Howell  |  
|   Voltage  |    220-240 Volt  |  
|   Wattage  |    300 W  |  
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