Findong Latias

It is kind of weird but you find him in rout 117,111,118,120,121,123,122,126,127,128,sootopis city,134,and 110. My sejeson is to not try to look for him wierd right. What I am trying to say you find him on acidant not on perpose

After you beat elite 4 ,when you go down stairs at your house,you mom will ask you red or blue.Answer it red for latias
Or blue for latios.If you choose red ,latias will randomly appeared in sea(i think)or if you choose blue,latios will randomly appeared in grass
Or long grass(coz I found latios in long grass ) hope this help

After you beat the elite 4 you tell your mom that the thing is red and youll fin it randomly on the route the guy on top said, catch it with a master ball if you got one

First, you need to randomly encounter it. Then, you can see where it is on your Pokedex. Every time you cross the city/route lines or battle a Pokemon, it'll move. (Sometimes, it doesn't.) Keep going until you're in the same area as it. Then, put a Pokemon that's close to lvl. 40 (not lvl. 40, it has to be under) in your first slot. Make sure the wild Pokemon that are in that area are lower than that one. Then, use a repel. You'll eventually run into it. As soon as you see it, it'll run away, so make sure you have a fast Pokemon with Mean Look or Hypnosis. Good Luck!