Once you reach the last of the Sevii islands and solve the puzzle in Tanoby Key (the cave), the unowns are unlocked.

Once you have them unlocked, you'll find that the Unown question mark () and the Unown exclamation point (!) Are impossible to find, even if you're looking in the right place.

The question mark Unown is in the Monean Chamber (to the far right) and the exclamation point is in the Viapois Chamber (To the far left)

To find them:

(1) Walk to the back of Viapois or Monean

(2) Activate your Item Finder (It will say "...Nope! There's no response!"

(3) Walk a few steps and it should show up.

I recommend using a Repeat Ball. Works without a fight every time. Keep in mind that this cannot be done more than once for each one. I haven't tried this on LeafGreen so if you try it, please tell me.