See if you can measure the battery tube, if it’s 140mm long you can use protected batteries, which are recommended for flashlights in serial configuration, if it’s less you need to use unprotected batteries. In both cases you need button tops.
As to where to buy in the UK FogStar has a good reputation among vapers, for more recommended shops see Massive post on where to get lithium-ion batteries everywhere in the world! Look at Post 2-5 for info, And Canada Post 123 - #3 by BlueSwordM