Since neither Hedestad or Hedeby Island are real places in Sweden, we will never find them on a map. The best we can do is track what Stieg Larsson says about their general location.
Both places are about an hour's drive north of Gävle. This map depicts the location of Gävle, which is north of Uppsala and Stockholm.
Click on the image for a slightly better view.
Map image online, courtesy Wikimedia Commons.