It depends which kind. You can find them anywhere!
i think you get it at mount. pyre
That item does not exist in firered.
snorlax givs munchlax
You put rock insence on a sudowoodo and you breed it.
You can find one on Route 38.
Yes I think so
Mt. Pyre in Ruby/Sapphire/EmeraldLost Cave in FireRed/LeafGreenRoute 204 in Diamond/Pearl/Platinum
Pokemon Ruby has Pokemon that Pokemon Sapphire does not and Pokemon Sapphire has Pokemon that Pokemon Ruby does not.
You give it to your Pokemon and if you battle a trainer, you get double the prize money, like a luck insence
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.