The national mapping agency of Greece is part of the military, Hellenic Military Geographical Service or in Greek, Γεωγραφική Υπηρεσία Στρατού.
According to a few sources like Mark Dubin's The Dodecanese and the East Aegean Islands, large scale topographic maps are not available.
Note: Based upon working in the GIS (geographic information system) industry, countries that have mapping/geodetic agencies strongly connected to the military tend to have tighter controls on large scale maps and geodetic information.
Topographic information, including elevations, locations and identifications of buildings and roads, and bathymetric information is considered sensitive and of possible use to foreign powers.
Update 06 December 2020: a comment by gerrit prompted me to look around again. East View Geospatial has 1:50000 scale topographic maps. The dates range from the 1960s through the mid-1990s. Note that this is a reseller, not a Greek government or company site.