General:
* Distant
* Remote
* Far away
* A long way off
* Out of reach
Specific:
* For location: Across town, out of state, overseas, on the other side of the world
* For people: Unreachable, inaccessible, out of touch
* For things: Unavailable, out of stock, scarce, limited
Informal:
* Miles away
* A world away
* Not in the same zip code
The best synonym to use will depend on the specific situation and the desired level of formality.