Where can I find isogloss maps for Mandarin?
The shapes of Isogloss maps are generally identical to those of dialect maps. So I think this could give you some clues.
 

Below is a sample for isogloss maps(from http://www.yupoo.com/photos/9919/7485900/)

More of them:





Suggestion: For more information about isogloss maps, visit http://image.baidu.com/ and type in "同言线". Further reading: http://book.kongfz.com/15325/226467644/ This is a book about chinese linguistic geography. For more books in this subject, search "汉语方言地理学".