See if any of us "Septics" are nearby, you probably could make a trade for them, or some units have them to spare- they might just give it to you. theres also the PX - Cp. Liberty @BIAP has them dirt cheap, I used to see RAF types in there.
Got mine of e bay, cheshire fishing tackle, or the seller is royal postal, put out a searrch in militaria, they are an easy to find item, they also do the proper surefire batteries 12 for approx Â£15, anywhere else they are around a fiver each, hope its a help
quick related question to the Surefire thing; I have had a surefire for a while and so far very reliable, but with new batteries in, it suddenly stopped working, so i guess the bulb has gone and needs replacing... as i bought it at a yank PX many moons ago, where is a supplier/ retailer in the UK where i can get a replacement bulb for it? Cheers