How do you get a female starter on Pokemon pearl?

Unless you hack the game, it's just luck of the draw. The ratio of male to females is 7:1, specifically so that it is harder to breed them (though you can use Ditto), though it is possible to get a female starter, but only by chance.

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A way of "increasing" your chances is simply saving the game at a point right before the cutscene with Professor Rowan and keep restarting (aka SR) until the desired gender comes.