Pedometers can be purchased online and in-stores. It is important to get a pedometer that matches your needs. There are various types of pedometers and making sure you are purchasing the right kind for your needs will make your use of one a lot more enjoyable.
Knowing what type you require will depend on your goals, what you want to track and how you want to track it. For example, if you want to track every step you take you will likely be better off with a very low cost pedometer that doesn't have any technology in place to deliver increased accuracy in the counts.
Whereas, if your goal is to lose weight, get fit or have an accurate count of your steps during the day, you will likely want to spend a few extra dollars and pick up a Pedometer that has some sort of False Start mechanism to avoid counting excessive phantom steps when you aren't actually walking.
Once you have established what level of accuracy you want, then you need to decide on what you want tracked. Meaning do you only want to track steps or perhaps you want to also know:
How many miles you walked each day?
Provide you an estimate of the number of calories your burned while exercising?
Do you want to know how fast you were going?
Do you want to know how long you exercised for?
Do you want to people to look at past day's performance?