Obtain a cell phone with a GPS chip. Most newer cell phones, including the BlackBerry, Motorola iDEN, Boost Mobile, Windows Mobile and many others, already contain GPS chips.
Some phones, especially older phones, do not have the chips installed. Find out from the manufacturer if the phone will take a GPS chip. If so, the chips can be bought at cell phone stores, from the manufacturer or over the Internet. Install according to the manufacturer's instructions.