To find legendaries, you need to sometimes be a champion. Legendaries are hard to find so there is a chance to find it anywhere. Some legendaries are in different areas and you can only find it there besides the Pokemon Roulette.
Hunch Areas Edit
There is a guy with the wings in the last house, saying that there is rare pokemon at ____!
After that, you can only find these legendaries in hunch areas.
- Manaphy (now Pokemon Roulette only)
Pokemon that you can find anywhere Edit
You can find Mewtwo, Mew, Groudon & Kyogre (with Rayquaza in your party), the rest of the Swords of Justice (need Keldeo), Entei, Suicene, and Raikou, Latias and Latios.
You can find the rest of the information at http://project-pokemon.wikia.com/wiki/Legendaries.