in tall grassy areas
it might take a while though but its worth it

you have to win da battle tower and da prof lets you chose

I found Entei below Ecruteak City, in the grass where route 37 was located. Raikou is in the grass under Blackthorn city, the grass to your right to be exact. Just walk in there but be patient. Walk around until you find them. You may use super repel or max repel and bring pokemon below 40.This is really true! Just be patient in walking around. I used masterball because its super easy to catch with it and it can be cloned so you shouldn't have problems in completing your pokedex.

they randomly roam around johto(i think its johto in crystal right?), the only way to track them is to have fought them before then you can see where they are but they move quickly so it could be tricky, if you are going hunting for them bring somone with hypnosis, sleeppowder(really and sleeping or battle trap move)
HAPPY HUNTING AND HAPPY HOLIDAYS 2007(soon to be 2008) NICE

avoid using fly this will make them go randomly anywhere