Those parts of the world under influence of or with closer relations to the former Soviet Union were mapped by Soviet military cartographers. After the wall came down these maps were made public.
One of the most comprehensive collections of the maps can be found here:
http://poehali.org/maps. An English user interface is available. There is also a commercial site associated with this collection, but download from poehali is free.
Available scale - depending on region - is 1:500000 to 1:50000. The maps are in Cyrillic, of course, but offer excellent topography.
All maps should be in the S42/40 Pulkovo Krasovsky coordinate system. In TransDEM, use 3+1 point georeferencing plus image editing (crop margins, reduce resolution, and/or apply transparent margins).
As these are scanned paper maps there will a bit of distortion occasionally which TransDEM does not fully compensate.
For some areas of our planet, particularly the more remote places, these maps are still the best you get.