From what it seems like you are asking it is the 8th gym leader, and she is found in dragons den behind the gym. After entering search for a whirlpool (required HM) and pass through it and head to the back of the shrine and enter it.
" Be wary of this answer because it may not be so accurate ".
In Dark Cave.
By beating the gym leader Clair in Blackthorn City.
You can find it in the Dark Cave.
curmen cave
You find a misdreavus inside cliff cave
The cave is called Dragon Cave and it's situated in a lake behind the city the gym is in.
To find Sandshrew, you have to go to the Union Cave. Nowhere else.
you can get mareep in between the violet city and union cave
The tower
In her Gym
The cave entrance collapsed in that game, so it can't be entered. Sorry.
I found mine in cerulean cave