Well, actually you need only one ball, but that would've sounded weird in the title.
What you have to do is to take Dog's ball to Ravenholm. You get it in the end of Black Mesa East. You have to be careful, though: if the ball touches water or fire, it explodes.
Now, instead of attacking you, the zombies attack the ball instead. How simple and handy :)
Keep the ball alive, and you can just walk past most of the zombie hordes.
Even the super zombies find the ball fun to play with.
Too bad that the ball had a powerful ally.