Every laptop company has different batteries, and many laptop companies have different batteries for different models. So how to figure out which is yours? Well, you could buy it directly from the company, which means you'll probably pay a serious premium (Dell, for example, has been known to charge four times more than ones you can get elsewhere). Unless you have to, why pay more?
1 Obtain the model number of the battery you need. If you're simply replacing a battery, just look at your old one, and it should be there. If not, log onto the manufacturer's website and look up your model, then its accessories. Find the battery you need, and make a note of the model number.
2 Establish which battery you want, if you have options. Some laptops have extended life batteries, and some have batteries that charge quicker than others, both of which cost more than your garden variety battery. You can determine what is available to you by (again) visiting the manufacturer's website. If more than one battery is available, it will tell you. You might want a longer-life battery if you spend a lot of time "off-cord," or working on planes or in airport terminals. Quick-charging batteries tend to be a luxury, however, some laptops won't fully charge without several hours of charging; take that into consideration when deciding which to purchase.
3 Set the market. When you visit the manufacturer's website, note the prices for the batteries. This will establish the high-end of what you might have to pay, and allow you to compare. Take note of the model numbers for each of the batteries you might be deciding between, since you will want to make sure that you get exactly the one you want, and not a knock-off.
4 Comparison shop to determine who has the lowest prices. Buyer beware, however...there are a lot of small electronic websites that promise one thing and then send you the Japanese version of the battery, which may or may not work for your computer. If it looks too good to be true, it probably is.
5 Compare the prices to the price of the manufacturer. If it's close, go with the manufacturer, since you know for sure that whatever it sends you will be the perfect thing for your computer. Keep in mind that there is a slight, but very real, chance that loading the wrong battery into your computer will damage or even completely wreck your laptop.