Wrap it around your waist halfway between your ribs and hips, making sure to measure against bare skin. Check the measurement, and write the number down. You can also use a string if you don't have a flexible tape measure. Wrap it around your waist, and mark the point where it crosses itself. Hold it against a flat tape measure or measuring stick. You may be able to find a waist trainer at high-end sporting good stores, since some people wear them while working out. However, your best bet is likely to be online. You can find trainers at places like Hourglass Angel, Ann Chery, and Yianna. Typically, waist trainers start at $50 USD and go up from there. Even if the website displays your typical size, it's a good idea to compare your waist measurement to the sizing chart. Getting the right size for a waist trainer is more important than with other clothing. When in doubt, go up a size.

Summary:
Measure your waist with a flexible tape measure. Find a waist trainer online. Match your measurements to the perfect size.