Location: In a cottage waiting for my mansion over the hilltop!
It is randomly found in the grass or water in Kanto, but not in the Sevii Islands or Caves. Some people look for Suicune for weeks, and never find it. It is a very hard to find Pokemon.
Next time you have a question, ask it in Simple Questions. It helps us reduce spam, so it helps us help you. The link to the this overly helpful thread is in my sig, and in the group of stickies near the top of the FR/LG (GBA) thread listing.