Laurie - here's a recent thread:
http://www.brickboard.com/AWD/index.htm?id=933469
From your description it sounds like you may have LED's, not light bulbs. LED's typically are opaque cylindrical plastic with a spherical end where they illuminate. In the old days the plastic was the same color as the light produced (red and green LED's). Today, the plastic is frequently clear or white, even though the LED may produce another color (blue for example). There are two pins, and usually the plastic case has a "flat" spot next to one of the pins for polarity. The replacement must have the pin with the "flat" spot connected the same way. If you are looking for an LED, RS carries them. They do come in different brightnesses, and I'm not sure someone can give you the info on that. The variety of physical sizes is fairly limited. As mentioned in a separate post if this is an LED, there is a current limiting resistor that you will not be replacing, which controls the current you are able to draw. So it is a bit of trial and error. You don't tell them "I need a 12 volt LED", because that resistor will be dropping several volts. LED's operate in the 2 to 4 volt range, give or take. Most will probably illuminate in the circuit, it is a matter of if you like the brightness that results. RS should have miniature bulbs if what you have is a standard light bulb. Good luck.