Söderköping is easy to find. E22 runs through the city and it is just 17 km to Norrköping where the airport and railway are located. From Söderköping you can take the bus to Norrköping and Valdermarsvik. Coaches stops here and from Stegeborg and Skenäs it's possible to take a road ferry over the bay. The road ferry is free of charge.
The local buses that goes to and from Söderköping are number 45, 46, 450 and 459.
Please notice that bus number 46 only has one stop in Söderköping, the stop is called "Östra rydsvägen" and also that bus number 45 does not stop at the train station in Norrköping. It's not possible to pay with cash on the local buses.