If you're talking about availability of a resource or person:**
* Unavailable
* Occupied
* Busy
* Engaged
* Out of stock
* Booked
* Not available
If you're talking about availability in a general sense:**
* Unreachable
* Inaccessible
* Unobtainable
* Limited
* Scarce
Example:
* "The meeting room is available at 10 am."
* Opposite: "The meeting room is booked at 10 am."
* "The new phone is available in black, blue, and red."
* Opposite: "The new phone is only available in black."
It's important to consider the specific context to choose the most accurate opposite.