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If you've spotted them once, use the Pokedex. Their location will always move from one route to the next, but you can use the 'dex to track them.
If you haven't, just wander around various routes (with the radio tuned into Pokemon March, if you can) and start praying that the gods of luck happen to smile upon you on that day.
Also, bring either Pokemon that can put them to sleep or a pair of Master Balls. They flee, and they know Roar.
Aside from sharking it, you can't. Getting the GS Ball was a Japanese-only event (as in, yes, event, so you can't just get a Japanese copy of Crystal and expect to be able to trigger the process that would result in a Celebi) that made use of the mobile adapter that the game came with, so for the English version, there's absolutely no way to get a GS Ball.