The only game you can ”catch” Celebi in is Pokémon Crystal. All other Celebi are event Pokémon. If you want the SS/HG Event Celebi, so you can trigger the time travel event, then there are plenty of ways to get it depending on your platform, Emulator or Console.
Emulator is easy you can just use PKHX to edit your save and add that specific event Celebi to your PC in one of the boxes. This is the easiest.
If you’re playing on a Flash Cart on a DS you can just use PKHX to edit the save on the micro SD card the same way you would with an Emulator. This is pretty easy, but requires a Flash Cart which run about $10-$20 depending on the model
Lastly if you’re playing on an official copy on a DS then you could do 1 of 2 things.
First you could use an SMS4 or an R4i Save Dongle to edit the save from your official copy in PKHX as before. They run at least $35, so this is expensive and a bit more difficult.
Second if you’re running a 3DS with custom firmware there’s a program called PKSM (Pokémon Storage Manager) that allows you to transfer Pokémon from a box on an emulation save to the PKSM box using PKHX. The Pokémon in the PKSM box can then be sent to a box in any Pokémon game on your 3DS. This is more advanced however there’s a Youtube tutorial on how to do it so it’s not too difficult.
Unfortunately if you’re running an official copy on a regular DS then you’re only option would be to trade for one.
