This should help you understand them and how to find one. You can usually find them at places like sports stores (Academy or Bass Pro Shops, as examples) or even walmart. I got a nice one for just $50 at Walmart, though it has its quirks. www.sparkpeople.com/resource/fitness_artic
To be guaranteed the most accurate reading, you should get one with a chest strap, and it will need to allow you to enter age, gender, and weight.