The lap timers that come with new stock bikes are more appropriately called stop watches. You push a button to start it, then push it again when you think you are crossing the starting line. You don't need any transponder. As you can tell, this isn't very accurate.
You want an infrared lap timer. These systems require the main receiver unit you install on your bike and it will also require a track side transmitter/beacon. I don't think most tracks have track side beacons set up, but apparently, there is usually at least someone who always brings a transmitter (they all work with all receiver units) to the track, but you can spring the extra $50 for your own to be safe.
Another kind of lap timer is magnetic. This will pick up magnetic strips built into the track. These are really nice if the track has these, but I don't know any tracks that do, and I don't know how you find out either.
The ultimate kind is GPS. You don't have to rely on a trackside beacon or strips built into the track. Satellites figure our where you are and you can set up as many checkpoints as you can dream of. Down side is the huge price.