Delcatty can be obtained in HG/SS by evolving from Skitty, it cannot be found in any area.
To get Skitty in HG/SS, you need to use the Pokewalker and unlock the Hoenn Field Pokewalker route and find one there, and you can trade it from D/P/PT.
like PB said only i will add to it, you can get one by migrating from RSE
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