The SS Ticket in Pokémon Silver is a plot device. It represents the opportunity to travel to the Sevii Islands, which are a separate area accessible through a special event. You don't actually find the ticket itself, but rather you earn the ability to travel to the Sevii Islands by completing certain tasks.
Here's how it works:
1. Receive the "Ticket to the Sevii Islands" from a Game Corner employee after catching a specific number of Pokémon.
2. Board the ship on the dock near the Game Corner to travel to the islands.
3. Explore the Sevii Islands and encounter new Pokémon and trainers.
While the "SS Ticket" is not a physical item, it serves as a symbol of your achievement and access to the Sevii Islands.