The first problem which you have to solve while buying a battery is to find a proper model which fits namely your laptop. Sometimes it can be difficult. There are no universal laptop batteries. Each laptop manufacturer works out various batteries which sometimes do not fit laptops even of the same series. So, the battery for Dell Latitude D630 will not fit Dell Latitude D631. Therefore for a start you should find out the exact name of your laptop model. Besides, each battery has a manufacturer part number which also allows identifying the battery unequivocally.
So, if you know your laptop name or a manufacturer part number for your battery you are ready to search for a new one. I must tell you that it will be better, if you search for the new battery according to the laptop name as it can appear that there can be variety of batteries for your laptop which can differ in price, capacity, manufacturer and other parameters. If you search for the battery according to a manufacturer part number, you will find exact analogue of your battery and miss other more favorable variants.