I'm lucky enough to have a partner who is involved with a theatrical group, so I can scrounge red lighting "gel" when I need some. I use it on my rabbitting lamps as the bunnies seem less spooked by red light than white and the red light does not mess up my own night vision so much either.
If you do want to redden a more powerful lamp like this (not a torch, but something thats running on perhaps 12 volts with a 30 or 50 Watt bulb) beware of the Babybel / biscuit wrapper / sweeties packet idea. Most of these plastics are too easily melted and last no time at all on the front of a powerful lamp due to the heat they give off. Proper lighting gel also gives a "purer" red than others, that's why it's used for lighting effects in theatres and on disco lights.