Believe it or not, go to the fishing dept. I found that those boobers that light up for night has a small colored insert that goes in them. I buy the replacement insert light. You turn them on/off by pushing/pulling up the bulb. The package says they last up to 25 hours. They come in 3 different colors yellow, green, red. They are small enough to drop down a .22. I use them to check out my .58 cal Enfield rifled musket. They are small enough to carry in your pocket. I even take one with me to gun shows to check out bores of weapons I am interested in. Cost about 4 bucks American.