Go to route 66 kanto region use poke radar in the grass where the kid with headband is once you do that go in to the wiggling grass patch closest to you and catch. You catch it at level 70 so be careful. If you use ho-oh against celebi it should have at least 6 hp left but make sure you save before you use the poke radar because there is only one celebi in each game so if you kill it without saving you are screwed.
P.s. If you do wireless worldwide trading go on and look for colby because I can give you lugia, ho-oh, darkrai, arceus, regice, registeel, regirock, regigigas, dialga, palkia, garantina, infernape, monferno, chimchar, empoleon, prinplup, piplup, tortera, grotle, turtwig, dragonite, shaymin, ryquiaza, groundon and kyogre.
I'm not lying.
P.s. If you do wireless worldwide trading go on and look for colby because I can give you lugia, ho-oh, darkrai, arceus, regice, registeel, regirock, regigigas, dialga, palkia, garantina, infernape, monferno, chimchar, empoleon, prinplup, piplup, tortera, grotle, turtwig, dragonite, shaymin, ryquiaza, groundon and kyogre.
I'm not lying.