You cheat or trade from a guy who cheated.
after beating league champion, compliting first pokedex (not jirachi and deoxsys) then random in woods.
I don't think you can but if you have Deoxsys on Pokemon fire red leaf green or ruby/sapphire emerald then you can transfer it to your Diamond
if you have vba emulator go to www.doperoms.com use next words: Pokemon psychic(sapphire hack) there is three starter pokemons lugia mewtwo and deoxsys. and more legends in rout 103.
With an event you can get deoxsys here
Pokemon Ruby has Pokemon that Pokemon Sapphire does not and Pokemon Sapphire has Pokemon that Pokemon Ruby does not.
There is no Pokemon mansion in sapphire.
He is not in Pokemon: Sapphire. You will not be able to find him. He is in Pokemon: Diamond, Pokemon: Pearl, and Pokemon: Platinum. You will definitely have trouble finding him in Pokemon: Sapphire.
Evee is not availiable in Pokemon Sapphire.
Any Pokemon in sapphire but you have to migrate
you wait until the rocket launch is at exactly 50 then the guy will send you onto the moon
You cannot get Regigas in Pokemon Sapphire as it is from the next generation of Pokemon
sinnoh Pokemon won't appear in Sapphire version!