Check your existing cutter. Most can be changed from left to right simply by moving the blade to the other side.
Martelli makes a rotary cutter especially for lefties; I got one for my sister. This is also an ergonomic cutter that is easier on the wrist. Here's the website for it, and they're currently on sale onine:
http://www.martellicatalog.com/mm5/merchant.mvc?