You have to go to hearthome city and talk to fantina and she vil give you celebi
How Do You Get Celebi On Platinum Without Migrating?
You get it by catching all the unknown including the 2 special ones you get after catching all the unknown in solaceon town then I believe you have to translate the writing then follow what it Say then walk for like some time and done but I really doubt this works I Just heard bout it and sounded realistic so yeah might not work sorry :* also I hear you can do this also for mew but I am pretty sure it won't work lol sorry
Wait for a celebi event or you can trade.. I have one and can give you one for an Arceus. .