it's in mossdeep city
in mosdeep
In emerald it is at the space station
scotopolis city
its 70
In mossdeep, go to ure right then up
the space station in mossdeep city
With an event you can get deoxsys here
They will come and try to steal the fuel from the space station. This takes place after you have beaten the Mossdeep gym leader
it is in lilycove it is in mossdeep in the house beside the space station
In mossdeep city near the space station
Emerald station
there is no mosdeep in diamond that's only in ruby, sapphire, or emerald