If your goal is a just a photoelectric sensor that triggers a logic signal on an Arduino, you can do this with a few dollars worth of parts, fed from the 5V regulated power supply out of the Arduino.
You'd use an IR LED as the light source. Then you'd use an IR phototransistor as the sensor. Feed that into the gate of an NPN transistor. Then you'd feed the output of that to the Arduino.
The total list of parts: A high output IR LED. An IR Phototransistor. A general purpose low-power NPN transistor. 2 or 3 resistors. Plus a reflector or some reflective tape if want the sensor to be all together in a box. Plus a breadboard or prototype board, some jumper wires, and a project box.
The company HiViz.com sells kits and assembled circuits designed to trigger cameras and flashes, but it would be trivial to adapt it to control a logic input on an Arduino. (The camera and flash circuits use an opto isolator to isolate potentially high voltage flashes from the other electronics, but if this is purely powered by the Arduino, you won't need that.)
I just built a prototype circuit like the one I described using just an NPN transistor, a photogate, some resistors, and a breadboard. If you're interested let me know and I can figure out how to create and upload a schematic. (I haven't tried using the web-based schematic editor before, but I can probably figure it out.)