The Virtual Console release of Pokémon Crystal allows you to obtain the GS Ball after entering the Hall of Fame. Giving the ball to Kurt and waiting one day will let you place it in the Ilex Forest shrine (and thus battle Celebi). This event is not time-restricted, but it's no longer possible to download Crystal from the 3DS eShop.
GS Ball (Bulbapedia)
Celebi was only available in Crystal from an event, years ago when the game was released. You got a "GS Ball" apparently and it made Celebi appear in Ilex Forest.
You can't get it now in Crystal, but there may be events to get it in HG/SS.
