How do you get a pika in Pokemon XD gale of darkness?

You can't get a Pika in Pokemon XD: Gale of Darkness. Pika is not a Pokemon in the game, and the closest thing you can get is a Pikachu.

To get Pikachu in Pokemon XD, you need to trade it from another Pokemon game. There is no way to catch or obtain Pikachu within the game itself. You can trade with other players or use a link cable to transfer a Pikachu from a compatible Pokemon game.

Remember, to trade with another player you need to have a compatible game and a link cable. You can also use the Pokemon Bank service to transfer Pokemon between games, but this requires a paid subscription.