I need a plunger! Where can I get one?

I'm currently playing though the Milkman level of Psychonauts, and have found the sign and the flowers and the book.

Now I have to find a way into the sewers, and judging by the appearance of these sanitation engineers, I'll need a plunger:

The problem is, I'm sure I've searched all the houses and every fridge I can find, and still no plunger! Where can I obtain a plunger?

The plunger is inside the post office. It's not obvious how to get in.

But you don't need any items. Just one of your powers.

You need to trip the alarm to get one of the agents inside to come out. At that point you can back off, but use Vision/Clairvoyance on the agent as he's going back in. That will reveal the keycode they are using. After that, just walk up and use the code yourself.