In order to find ! and ? Unown, you must first complete the Ruins of Alph's four puzzles, and unlock the other 26 Unown.
The first to solve is the Kabuto puzzle, close to the start of the game. This will yield access to Unown A - J.
The next to hunt down is the Aerodactyl puzzle, accessible once you obtain Surf. Solve it to get Unown R - V.
Third is Ho-oh, also available after you get Surf. Completing it allows you to catch Unown W - Z.
Finally there's the Omanyte puzzle, which is available with Surf and Strength. Solving it will allow the capture of Unown K - Q.
Once you catch every letter of Unown and visit all the hidden back rooms, head back to the Research Center and talk to a scientist for more research papers. Inspect the back panel at the entrance of the ruins with a scientist to find another room. When you're able to explore the room freely, you'll be able to find the ! and ? variations of Unown.
For a slightly more in-depth guide, read the Psypokes article.
