Why is the EV3 Ultrasonic Sensor (45504) out of stock?
Balanc3r is a good alternative for gyro-boy:
http://robotsquare.com/2014/06/23/tutorial-building-balanc3r/

However, why do you conclude that the 45504 sensor is out of stock?
In my region (Belgium) it can be ordered from the lego website and is listed as "Available", also it is contained in the core educational set (45544) which is certainly still available from various sources...