terryking228:
I think many of those sensors are Infrared beam modulated at 38KHz. You could build you own with IR LEDs, a 555 timer to drive them, and typical 38KHz receivers like these:
http://goo.gl/2uwYM
Do those allow a continuous 38Khz signal? Most IR receivers require some period of off time in the signal (and a 555 timer alone would be insufficient).
The only IR receivers I'm aware of that allow a continuous signal are these "Presence Sensor" sold by Vishay (TSSP57038, TSSP6038, TSSP4038, and TSSP58038).