Hi, I want a simple alarm system which will ring if any of three doors is open while the alarm is armed (it is not for a car but for a boat). I would like to have four transmitters/fobs, and to be able to replace them easily in the UK.

I have looked at the Pyle PWD701 for example, but it only has two transmitters. When I asked the company where to find more transmitters, they said: "Just purchase another PWD701 system to get two more transmitters". This is expensive and seems nonsensical to me.
Is there any alarm system out there, that I can easily find cheap transmitters for?