In one of the round garbage cans beside the underground tunnel's gate. You have to dig through some garbage before you can find the tickets. After that take them to the old man in the internet cafe. (If he doesn't pay attention, take a green balloon from the Clown Store to the kid in the Country Side, then try again.)
You can look around the island all you want. The "Underground Tunnel Tour" requires tickets. (see related question)
They are in one of the two trash cans near the Tour entrance (under the garbage).
They are in the trash can by the tour guide.The trash can with the lid off
They are in a round trashcan to the right of the Tour entrance. You'll have to sort though the garbage until you find them. (see related question)
The Underground Tunnel Tour tickets are in the bottom of the second large round trash can near the tour entrance. You cannot use them immediately because they are not yours. You will find the owner of the tickets in the Internet Cafe on Main Street. But before going there, get a green balloon from the Clown Store and take it to the Countryside. The result will ensure that the owner of the tickets is not too preoocupied to talk to you.
In the bin near the tunnel entrance.
Look in the round trash cans to the right of the Tour entrance. Move the trash out of the way until you find the tickets.
The two tour tickets are in a round trash can to the right of the Tour entrance. Dig down through the trash to uncover them.
It is in the tunnel when your taking your tour ,but it is hard to get.
The first piece of paper in in the counterfeit banquet, the second is in the tour tunnel, find the rest
Find tickets at the bottom of the trash can and bring them back to the guy in the café. He is so greatful that you found them, he gives you one and you are able to get a tour and go back anytime. Hope that helps.
In the second trash can to the RIGHT of the tour place then go to the man in the internet cafe.