Rotary cutters come in diameters of 60mm, 45mm and 28mm, as well as 18mm, which I use for doll patterns. The one that I own is made by OLFA and costs about $13. A package of 2 replacement blades can be purchased for about $8. It features a sliding piece to cover the blade while not in use, so you don't slice your finger or anything else nearby!
You'll also need a self-healing cutting mat, to protect your sewing table from the blade. These can be found in the cutting or quilting section of your local sewing or craft store. I also recommend having a ruler to line up with the straight edges to help keep the blade nice and steady. Clear ones are great because you can see your pattern piece right through them, and the thicker the ruler is, the less likely your blade will slip.
I little background on rotary cutters: they were first invented in 1979 by the OLFA Corporation, headquartered in Osaka, Japan and founded by Yoshio Okada. OLFA were also the ones who invented the snap-off blade design that you find on most box cutters! Click here to find out about other neat tools from OLFA.