I can't find the circuit breaker switch to cut the electricity to my intermatic T10004rt1 so that I can replace the timer model T104. There is no circuit breaker for it on my house panel so I assume that there is one on the T10004rt1 but I don't know where it is. How do I cut the electricity to the panel so I can replace the timer?

Is this for a HW heater? 240VAC? What does it control?

The intermatic T10004rt1 appears to be some kind of pool panel and transformer and the timer would be on that same circuit. Turn off breaker for intermatic T10004rt1. Check for voltage with meter before doing any work. If you are not sure, kill the main breaker.