You can find these at hardware stores and some hobby shops. The hobby ones are usually a strong plastic while the hardware stores carry ones made out of metal and sometimes with two cutting wheels.
Hobby shops and light metal are pretty generic, but hardware stores will rate them for different sizes and thicknesses of pipes if you want to do heavier duty stuff.
Their use is actually quite simple. Just loosen the cutter until the pipe fits, then tighten it in - snugly, not too tight. Next, spin the cutter all the way around the pipe - you should see a faint line cut around it. Tighten a little more, and repeat until you cut all the way through.
Smaller pipes can be difficult to hold strong enough to spin the cutter around, so you may need to use tin snips instead.