Registeel can be found (but refuses to be caught) in The Lounge! Known Locations The Lounge Video Games Roleplaying Institute Art Corner QnA
I spotted a Registeel in the Roleplaying Institute however I didn’t have any Pokéballs so I couldn’t try and catch it.
I found SEVERAL in the Roleplay Institute. Encountered four, caught two. Was too lazy to get the others. Found Regigigas there, too. Be careful. He ate up almost all my balls...
Spotted Registeel in the Q&A section: Spoiler: Here I also found Shaymin there, but I didn't have a screenshot to prove it at the time.