Locating your alarm's transformer can require some effort.
First, you're looking for a (normally) white, beige or black box , plugged and screwed into an electrical outlet. Some older systems may have bare frame (without plastic covering) transformers, but that's the exception.
Check first the area in close proximity to the panel. Barring an easy find, it is possible that the unit was hardwired to the circuit breaker box, so look there for anything labeled "alarm" or "security".
Either way, it should be located on a circuit that isn't controlled by a switch. Some examples of where to look are:
The transformer can be located in any of these locations. I've only provided a few that I can think of, but you're going to have to put on the detective's hat.
* Image courtesy of Elk Products