I believe the bulb you need if its the low beam is a Phillips ds2. When you take it out it should have a part number on it. As with any other bulb, don't touch the glass part. You should replace both at the same time. Before you buy bulbs you should try switching them around to confirm it is not the ballast. If it is the ballast then you can find the manufacturer and part number on it. Im not sure which ones they are but they are probably oe for many different vehicles and can be found used for cheap on ebay.
06' wrx tr