Paper towns are towns added to maps to avoid plagerism; it's also reffered to as a copyright trap. The paper town in the novel where Margo is is called Agloe, New York and it is very much real. However, when looking for the town on the map below, it doesn't come up because again, it's techincally not real, but at the same time it is. Therefore, I used Rockland because it is quite close to Agloe. 

Margo decides to runaway here because no matter where she is, she is unhappy, so why not go to a place that doesn't exist? If it's not real, can she really be unhappy with it? Does she want to exist without existing? 

Quentin finds out Margo is in Agloe through an anonymous post that read “fyi, whoever Edits this––the Population of agloe Will actually be One until may 29th at Noon." Quentin knows it is Margo who wrote it because she had the habit of randomly capitlizing letters. Having found this out, Quentin and his friends go on a roadtrip to New York.