There are two tickets to the Underground Tour in the second of the pair of trash cans to the right of the entrance. But you can't use them because they aren't yours. Once you have taken a green balloon to the boy in the Country Side, you will find the ticket owner at the internet cafe on Main Street.
On counterfeit (or how ever you spell it) island you find the tickets in the garbage near to the right. once you are done that you bring them to him and you will get one of the two tickets
you find all the clues like the tickets in the garbage can
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.
in the trash can next to the tour entrance then bring it to the guy in the Internet Cafe.
Although the girl has sold out, you can find two tickets to the Underground Tour in the second round trash can by the Tour entrance. You should return them to their owner, who is at the Web Browser Cafe on Main Street.
Go on www.youtube.com, search in counterfeit island walkthrough. It might help.
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)
You go next to the tunnel, and the you click on the second trashcan to start looking in it to find two tickets. After you find them, return them to the guy in the coffee shop. He will give you one of them.
If you mean on Counterfeit Island, they are in a trash can a little past the tour guide. Click on it, remove all the crumbled up pieces of paper out of the way, and you will find the tickets at the bottom. After that, give him the tickets back and you can have one in return to go on the tour.
First go to skullduggery island,take your ship to a island called Keys World island and you will find the key of the chest on counterfeit island!;)
After you find the Underground Tour tickets, go to the Internet Cafe and give them back to the old guy there. If he is too busy playing internet games, you have to distract him somehow. As it turns out, this is accidental if you go to the Countryside and give a green balloon to the boy there.
Find out for yourself