What to Look For
Portable bike pumps come in several styles. Frame pumps are the largest and easiest to use, but require more space and frame mounting. Mini pumps are smaller than frame pumps, can weigh just a few ounces and can often fit into a backpack or saddle bag. However, these can be cumbersome to use and take a long time per fill. Decide how you want to carry the pump and how important weight is and select accordingly. When shopping for pumps, practice using it to make sure that it'll be feasible to use on the road or trail. Look for a handle that is easy to grip and a pump that provides for a quick, steady pumping motion. Double-action pumps fill on both the upstroke and downstroke of pumping. Carbon dioxide inflators are light, small, fast options that will help you get home, but carbon dioxide is not reliable for keeping tires full. For road bikes, make sure the pump is capable of providing the high pressures needed in your tires.
Check your tube type before purchasing a pump. The two main types of valves are Presta and Schrader. The Schrader valve, often used on less expensive bikes, looks like the valve on a car tire--short, black and thick with a metal end covered by a plastic cap. Presta valves are long, thin and metal and have a screw-on nozzle at the top under a plastic cap. Some pumps work only for Schrader, some for Presta and some can convert for both types of valves.
Recognize that a portable pump doesn't replace a solid floor pump for home use. Portable pumps are designed for emergency repairs but floor pumps will deliver much quicker, more convenient performance for home repairs and maintenance.
Where to Buy
You should find a good selection of portable pumps at your local bike shop. If you're not picky as far as weight and design, you can also find a lesser selection at sporting good stores and department stores that carry bicycles and accessories. Online, you'll find pumps at bike retailers like Bike Nashbar and at outdoor sporting good retailers like REI.com.
Portable bike pumps range from around $15 to $80. More expensive pumps in that range use materials like carbon fiber to cut weight. You'll find plenty of mini and frame pumps between $20 and $30.
If you choose to use a CO2 inflator, you'll need to purchase CO2 cartridges.