I got mine when someone came up to me at a street fair and said, "I have a light that fits that" when I was on my Raleigh. Hooked up later and he gave it to me free!!
You probably will need to check Ebay and particularly Ebay in the UK where they are much more common. Mine is an "Elite" and runs 2 - D cells. I did put an LED bulb in it but it still pretty much is for looks or be seen .
VINTAGE RALEIGH BICYCLE LIGHT (LAMP) & HOLDER FITS HUMBER RUDGE ENGLISH 3 SPEED | eBay