You must have beaten all gyms and all elite fours to get legendaries.
Here are some really good places to find legendaries.
1. in the cave map (not mansion) go to the higher ground which is sand with no water on it on the same map it has the computer trainer steven. There i found darkrai,arceus,mew,dark mewtwo, registeel,regirock,dratinice,regice,groudon and some other guys all in the same day.
In the mansion beside the mew statue (looks like marowak) there are many legendaries. my friend saw azelf there on his first shot.
Eevee-go to the grass map that is right of the grass map with the entrance to the electric place
The first one is the best