Vehicle headlight bulb numbers are typically located in the owner's manual. If the manual can't be located, it can usually be obtained from the vehicle manufacturer's website or dealership. Most auto parts stores also have reference guides for looking up the appropriate bulb number for all of a vehicle's lights.
Many bulb manufacturers and auto parts retailers have a cross reference list of acceptable substitutes if the brand, and model number, of the lamp is unavailable for a specific vehicle. During installation of a new bulb, the bulb glass should never be touched by the installer because the oils on human skin can cause a bulb to fail prematurely.