It's random, but you can try something that I did for catching Entei.
Go to Celadon, near Celadon, to the right, there is a small piece of grass. Buy some max repels and go there, but, before enter in the grass, save your game. Then use the max repel and go into the grass, if Suicune doesn't appear, reset your Gameboy and try it again, always in that part of grass near Celadon, do it 'till it appear.
Good luck!