where do you get a serious pokemon in platinum

[pete]

were do you get a serious nature pokemon in pokemon platinum?

[Zidarose]

Serious is not a fixed part of any Pokemon, it is in fact, a randomly determined part of any given Pokemon. There are numerous natures any given Pokemon can have. To get a Pokemon of a particular nature, just keep catching them or try to breed it with a Parent who has the right nature and is holding an Everstone.

A complete list of the various natures can be found in this guide:
Pokemon Nature guide