Hi,anyone has some clues what kind of plug I must buy for this ozonator?
A very peculiar plug...
https://poolking.ch/Ozonator-P78B138/19248
Thank you

 

If it's just to plug it into the mains, I would cut it off and put your own plug on.

 

DigiKey Universal MATE-N-LOK
There are male and female plastics. Note the link should take you to 4-pin only. In your case pins 1 and 4 are not round.
The metal parts inside also come male and female.

 

Without at least some information as to the intended supply voltage the very best you can get is guesses. What is the intended supply voltage?? Like most amazon listings the link gave no useful information at all.
The blue and brown wires point toward an European power source. and so it may be that it is intended for either 120 volts or 220 volts. BUT with that style of plug it does not seem to be intended to connect directly to a mains source.
Isit intended to be a part of a system that we are not told about??