As Saz said, you cannot get the Incenses again in-game, there is no place to buy them. You will have to trade them from another game.
The reason that Incenses are needed to breed Pokemon in Generation 4 is because the baby Pokemon were introduced in that Generation, their "adult forms" were already available in previous generations.
It wouldn't really make sense to for example breed a Snorlax in Gen 3 and get a Snorlax, but breed a Snorlax in Gen 4 and get a Munchlax. So that's why the incense is required.