* Shiny Pokemon were not introduced until Generation II (Gold and Silver). LeafGreen is a remake of the original Generation I games (Red, Blue, and Yellow), which did not feature Shiny Pokemon.
* Shiny Pokemon cannot be traded back from later generations. While you can trade Pokemon from later generations into older games, the Shiny feature is specific to the generation the Pokemon was originally caught in.
What you can do in LeafGreen:
* Breed for Eevee: You can find Eevee in the Safari Zone and breed them for various Eeveelutions.
* Catch a normal Eevee: If you're happy with a regular Eevee, you can find them in the Safari Zone.
If you want to find Shiny Pokemon:
* Play Pokemon Gold, Silver, or Crystal: These games introduced Shiny Pokemon.
* Play Pokemon FireRed or Emerald: While not as common, you can find Shiny Pokemon in these games too (but you won't be able to get a Shiny Eevee since they're not available in those games).
Let me know if you have other Pokemon questions!