If you mean paras, you have to go to the Great Marsh after getting the national dex. If you mean wormadam, evolve a burmy, which you can find from putting honey on honey trees in the Eterna forest. It evolves at level 20. There is also a trainer who has one on route 214, by Veilstone city (the one with blond hair).
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Wormadon. Wormadon Is the number 46 on Pokemon diamon
Paras
According to the serebii.net pokedex you can get it in the safari game. (If you are talking about National Pokedex number 46, which is Paras).
on the sinnoh pokedex its the Pokemon wormadam
45=burmy 46=wormadam