Where do I start the treasure map quest
 

Where do I get the first map that sends me into the hunt

Just kill pyrats and bandits for maps. It's all the same, as far as I remember.

Really? I've been killing these things for a long time what's the probability to drop

Idk

Bandits have the highest drop rates.

You need a bag for the maps first, you can buy it during the quest for the god bow.
check: http://www.graalians.com/forums/showthread.php?t=29148