Thank you so much, it was really helpful Philipp Gläser

The German Aerospace Center recently released the TanDEM-X Digital Elevation Model for scientific applications with a resolution of 90 meter. --> https://geoservice.dlr.de/web/dataguide/tdm90/

http://loadmap.net/en

If you need old topographic maps (not DEMs) you can also use this resource

There you can find Soviet military maps (1:500 000 scale) and U.S. military maps (1:2 500 000 scale) of Ghana

Ozi Explorer's calibration is supported

Really appreciate your suggestions. They've come in handy Anton Romanov Tobias Ullmann