I built my own. 24vac coil so I can drive with like the valves I use.
Contacts are double pole rated for the line voltage here it is 240vac. Europe it is likely to be 220vAC.
I have a small water pump but decided to make the relay support a big water pump so contacts are rated to switch 10Amps, inductive load. element14, RS components, Digikey etc all sell these parts. Relay should cost under EURO10.
I then made up a box with a power lead feeding through an approved cord retention device. I mounted a female power connector on the box. I then ran a single pair cable to the relay coil, through a water tight grommet 10m long to connect to the Opensprinkler.
Wired it all unto meet safety code. The unit is rain water tight
It is legal for me to do this in Australia.