I am trying to find the milkman but I am stuck at the sectet agent town. I need to find a disguise and get down into the sewers, how do I do this?

I think the plunger is in the post office, but i'm not sure. Sorry, don't know it that helps at all.

You need to go to the post office, then try the access code box and put in any number so the alarm goes off. Go invisible and wait for the guard to come. While he is punching in his code, use Clairvoyance to see it. After he's gone, repeat the code to enter the building, then use Clairvoyance to navigate the dark back section to find the plunger.