Entei and Raikou start roaming Johto soon after that and you'll have to catch them.
Helpful Hint: Bring a Dugtrio with Arena Trap or a Wobbuffet with you. so they can't flee.
Suicune, on the other hand, doesn't roam Johto. Every time you see him, he runs away before you can battle, until you corner him at Kanto's Route 25, and you battle him.
EDIT: (Didn't see Ho-Oh, sorry.) Ho-Oh you get right after receiving the Rainbow Wing and beating all 8 gyms. He's in the Bell Tower, a place you can't get to until you get the Rainbow Wing.
You can also get Lugia when you go to Kanto and receive it from an Old Man in Pewter City. Then you find him in the Whirl Islands.