From Bulbapedia, the community-driven Pokémon encyclopedia.
Palmpona (Japanese: リーフタウン Leaf Town) is an anime-exclusive location in Johto. It is located between Azalea Town and Goldenrod City.
The town's biggest attraction is the Pokémon Swap Meet, where hundreds of Trainers come to trade Pokémon with others. Benny attended in the hopes of trading his Wobbuffet for another Pokémon. He reached his goal when Jessie accidentally traded her Lickitung for the Psychic Pokémon.
Another big attraction is a tournament to see who has the strongest Tauros. Ash had his Tauros sent to him from Professor Oak in order to compete in the competition. He was able to defeat the champion, and afterwards many people offered to trade him their Pokémon for his Tauros. Ash declined all the offers.
Palmpona appears to be named after Pamplona in Spain, which is famous for its annual Running of the Bulls competition. There is also a town in southern California called Pomona, where a large swap meet is held several times a year. 
Pokémon seen in Palmpona