Looking for Antique Maps of Northern Germany

[Lifthrasir]

Hi all,
I'm currently working on the map for a video game. It is supposed to represent the North Sea and neighbouring countries during the 1st half of the 13th century.
I'm trying to represent the coasts lines like they were at that time. I've already found what I needed for the Kent (England), West Frisia (from Flanders to Netherlands) and North Frisia (Germano-danish border).
However, I'm encountering a lot of difficulties to find old maps for East Frisia and Northern Germany, basically from the Dutch border to the Danish one, particularly around the Elbe river entrance, something like this one attached (area in the upper left corner but unfortunately, this map is from the 18th century).

I'd be very grateful if somebody could point me where to find such maps. Many thanks in advance.