At Radio Shack you can find a IR LED and Photo sensor pair for under $4. We used one on our Breakaway robot to detect when we had a ball. The key we found to ensure that the sensor didn't pickup stray signals was to mount it recessed in a hole so it wouldn't pickup ambient light and to make sure the emitter was aimed directly at it. Here is the page
http://www.radioshack.com/product/in...ductId=2049723 and the part number 276-142 it is listed as "This item is available at most stores" and does show up at the tiny store out in my neck of the woods.