For V3: They are sometimes available in the shop in your Tamagotchi. If you don't see any now, check back in about 4-6 hours*. You may not see any for a couple of days, but eventually they'll show up. You can use the time to get Gotchi points because you'll need them, since the tickets are expensive (several thousand Gotchi points each).
* Note: shop restock times are 3:00 PM, 7:00 PM, and 12:00 AM (midnight)
For EnTama / UraTama: You can purchase them at the market in the EnTama TamaTown / UraTamaTown web sites.