[Foster]

If you put a finger sized tube about 2" long over the stock Vex light sensor it can find a candle flame at about 2 feet. (Electrical tape works well) The tube blocks out the ambient room light. You'll need to mount the sensor at the height the flame will be and make sure that it is level with the floor.

You will also need to calibrate the sensor to the standard room light at the event so you can tell when the flame is present.

A black, matte finish inside the tube cuts the inner light bounce down. One thing you can try is to mount the sensor in the bottom of a black 33mm film can. You can then drill different sized holes in the cap to experiment with.

From radioshack.com:
CdS Photo resistors (5-Pack)
$2.99 Model: 276-1657

Would be a place to start if you want to try building your own sensor. Their are different sizes / types, so it will take some experimenting to get it to work.

Good luck!