For a while I've known this sensor to be obsolete and impossible to find; nobody carries it, and whatever happened to the ones produced - well, I guess they must be in use. Check out the comparison table here:
http://www.acroname.com/robotics/info/articles/sharp/sharp.html
(note, I have no affiliation with acroname)
While it was quite bulky compared to their other sensors, it could "see" for a good distance, almost triple the max distance you can get from their next best sensor.
So what happened? Why did they stop making it? What about those applications that used it; what are they supposed to do when they need a replacement?
So far, the only thing close to this that is cheap enough would be an ultrasonic sensor of some sort (likely something close to the Senscomp/Polaroid 6500 series, or maybe something from MaxBotix, Hexamite, etc). My only wonder would be comparison of beam spread between the two, noise on the return, etc.
There is such a gap that needs to be filled for a long-range distance sensor that doesn't cost an arm/leg (closest so far would be the Parallax laser distance sensor; then the Neato XV-11 LIDAR sensor)...