You can simply get another game (or a trustworthy friend's game), trade your King's Rock Poliwhirl to your other game, and trade back.
Some good things you can do is get the King's Rock Poliwhirl on your other game, and trade it to your actual game. That's a lot quicker. :3
Also, you might see some nice people in the GTS offering a Poliwhirl with a King's Rock. It's respectful when other people do that sort of stuff, so don't hold back if you see something like that. ;)
Hope I helped. :)