And what bulb should I buy for replacing? If you can tell me where to buy it, it would be even better. Thanks in advance for your help!

The easiest way to check is to have the car in a unlighted garage. Get something heavy like a brick and place it on the brake pedal. This will hold the brakes on and turning on the brake lights. Then go to the back of the truck and look to make sure the same lights are working on both sides (passenger and driver). If one is not open the trunk to get to it. If this does not work you may need to remove the plastic brake light cover on the outside, but it is doubtfull that you will have to do this. Normally there is access inside the trunk.

Remove the bad bulb, that will need only a 1/4 of a turn twist. The bulb will have the part number on it. If you can not read the number any auto parts store can look up the right bulb number.