I have read somewhere (I think the Arduino forums) that sharp no longer manufactures these sensors (though that can be hearsay). I do know however that they are quite expensive compared to much simpler solutions.
The simplest would be a IR LED - IR phototransistor combination:
Phototransistor on left, led on right
You would power the IR LED (remeber the limiting resistor!) and then use the phototransistor to pick it up. The voltage drop over the phototransistor will be proportional to the intensity of the IR light it picks up which is proportional to the distance the emitter and detector are from the reflecting surface.
Pointers (Personal Experience):
- It will take some experimentation and calibration to find the correct voltage - distance relationship
- The voltage will vary depending on the reflectivity of the reflector to IR light.
- The detector and reflector will need to be shielded from each other in order to gain a accurate measurement.
- This type of sensor is only reasonaly accurate. Keep it in mind.
- The detector is susceptible to outside sources of IR light, including the sun, incandescent light and florescent light bulbs.
There are many links on the internet on how to use said components. Take a look here: http://letsmakerobots.com/node/2907
Lastly, this method (I have found) is only accurate over small distances (not more than 20cm I'd say). For longer ranges you should consider ultrasonics or those fancy Sharp sensors.