Traditionally there have been two one at the 3 o'clock keyhole and one at the 9 o'clock keyhole. The latter is, I belive called "Johnny and the Playa Cruisers." When the theme camps are placed and the camp registered there will be notices here (this is a link!)
. Read through, see if one is available and contact thier contact person to see if they are looking for more people to help out. YOu might also be able to show up on playa.
You can also consider going free lance--put a sign on your bike and let people flag you down. or be the bike guy on your street (walk into camps and ask if people are having bike problems and tell them where to find you when they do) or just find a camp you like and see if they need a bike dude.
It's a valuable skill and if you want to come by the Eplaya Bar Camp and fix mine, you're welcome to!