Unfortunately, you just have to fill up your inventory with PokeBalls, and just catch as many Pidgeys and Rattatas and Zubats and whatever other Pokemon until you catch one. To be honest, though, don't catch a useless Pokemon instead of any other good Pokemon if PokeBalls are at stake. The chances are too low, the risk is too high, and Ditto isn't worth a PC box full of Zubats.
