In a pinch, you can swap a bulbs from either the fog, high, or low beams as all 3 (in halogen set-ups like yours) use the H7 55W bulb.
Additionally, I usually just go ahead and replace the same bulb on the other side. In most circumstances they were put in new at the same time and usually tend to go around the same time (w/in a week or 2). Of course, if you know a specific reason why, other than end of useful life, it failed you can just replace the one.
