That's actually because they don't want you to be able to get a hold of the map. Comiket is free to enter so they get their money out of you by putting the vender maps in an official event guide book that costs around Y4,000~Y5,000. They are sold in a number of stores and of course at the door. There's an electronic version you can buy now too. Think it's a CD.
I'm cheap though and just go blind lol
There's probably a pirated scan of the map someplace on the internet. Just not sure where.