You can never find the starters in the wild you can though find johto pokemon.
Kanto/Hoenn-migration. Also for Johto but you must migrate from Emerald, after beating the league in Emerald, you go to the Safari Zone and there'll be a new area. In the new area you can find Johto Pokemon including the starters (the starters are EXTREMELY rare to find though.)
cheats
Safari Zone.
Snubbull is a rare Pokemon that can only be found in the Safari Zone extension in Pokemon Emerald.
Rare. Safari Zone.
You can find a shuckle in Pokemon Sapphire, Ruby, and Emerald in the Safari zone.
You find evee on Pokemon Emerald at the Safari zone but in a specific location
Safari Zone(Encounter Rate is common)
Psyduck can be found in the Safari Zone while surfing.
Some where in the safari park :/ but I can't find it in there anywhere :'(
You can find one at the Safari Zone. It has a 5% chance of appearing.
There are no Eevee in Pokemon Emerald. To obtain one you would have to trade from another game.