As Ka`rla mentioned you get to them just by selecting the coordinates. So anywhere in the galaxy really=D You'd be lucky to find one without knowing it where it is, there are millions of combinations for the coordinates.
The word hidden is a misnomer. All systems are accessible, they are only hidden in that you cannot view a system containing just a space station from the galaxy map and the owners generally don't publicize the locations. If you fly to the system you will see them sticking out like a sore thumb.
No system is actually hidden anymore, but deep space is not a system. To preempt the next question there are no planets in deep space as they are all in systems.
Deep Space is any galactic co-ordinate that isn't named (like Tatooine is named etc).
As for hidden, people could put asteroid hideouts there, so not only would you have to be in the same system as the station, you would have to visit each square with a sensor 8 ship to see it (higher sensors and you can skip some squares, but would still take a long time ot check).