>The main path of Opelucid City is a big circle around the city. Going counter clockwise from the Pokemon Center leads to a house. Inside is a guy on the top floor surrounded by Tympole. He wants to see your Tympole.-Source
So he is located in Opelucid City, look around for him you should find him.
>The last important house may not be of much use to you right now. Head to a building just North of the Pokemon Center. A man with a ton of Tympoles will be on the second floor. If you happen to have a Tympole, his Tympoles will line up and sing for you. The only way to get a Tympole is to breed a Palpitoad or trade for one.-Source
You get no items, just a show.