It?s Restaurant le Wow in Hibernal Avenue.
The trainers there have pokemons around level 60, so make sure you?re strong enough before you pay the entry fee.
And remember to equip Amulet coin on your first pokemon.
Depending on how good you do it they will give you more or less Balm mushrooms, you can sell them for a big profit.
I haven't gone into that restaurant yet, but thanks for the tips... and the map! :>