Anyone know where I can find a basic historical map of the Soviet Union, circa around 1940?
Since your asking for maps from 1940 what are you primarily wanting to see, do you want to see the Soviet Union at its maximum extant right before the German Invasion (ie do you want to see the Polish Partition and the Baltic Republics as part of Russia?) During that time period maps of Europe were changing fairly often. The 1939 map below is a basic historical map of nearly that period, but between 1939 and 1940 maps of the Soviet Union changed substantially (Poland, etc...)
Here is a Soviet Political Boundary Map from 1939 - http://minchanin.esmasoft.com/maps/ussr1939/maps/01.jpg
Here is a 1939 Map of the Rail Way System in the Soviet Union - http://www.karty.by/wp-content/uploads/2011/10/railway_SSSR_schema.jpg
Here is a 1940 Map of the European Part of the Soviet Union - http://www.karty.by/wp-content/uploads/2011/02/karta_sssr_1940.jpg
Here is a 1940 Map of Different Ethnicities in the Soviet Union - http://www.karty.by/wp-content/uploads/2011/01/Ethnic_map_USSR_19411.jpg
Here is a 1939 Map of Industrial Zones in the Soviet Union - http://dot.mpei.ru/do/eres/hist/data/lesson_9_1_final/data/img/industrializaziya__sssr_karta.jpg