I have a few things to post about different kinds of batteries, so I'm putting them all in one thread. Some of this information should be useful to someone.
First off is the good old CR123A battery, AKA CR123, 123, 2/3A, and a few other names. It's a very common lithium battery the same diameter as, and 2/3 the length of a single A battery. They're 3.6 volts for the lithium ion, and 3V for plain lithium cells. If you use a tactical light like a Surefire, you probably use these. If you can't get your hands on them, you find any of these 2CR5 or CR-P2 battery packs for cameras on clearance sale, or otherwise have them in your possession, you can take them apart and use the 2 CR123A cells inside. Just cut them open, starting at the bottom so you don't short them out, and pull the metal tabs off them. One guy found them on sale for half the price of CR123A batteries, making it worth the trouble of taking them apart.