Where to find information about numbers of Mormons in France, Poland, Russia during WW2 I am interested in the numbers of members in the LDS Church in France, Poland, and Russia between 1935-1945.

Would anyone know what the amounts would be, or where it can be obtained from?
That is an interesting question. I too would like to know why you are asking, as I have never thought of this specific time period in church history.  Here is some information I could find by a few searches. I have quotes and I have attached sources as well if you want to dig further.
France

By 1930 there were only forty-seven French Mormons, whereas in the French-speaking areas of Belgium and Switzerland there were 344 and 280 Saints, respectively.
War would again interfere with missionary efforts in 1939 as missionaries were withdrawn from France with the outbreak of World War II. And even with the end of that conflict and the reopening—for the fourth time—of the French Mission in 1946, war still indirectly hampered proselyting efforts: with the Korean War and the need for U.S. soldiers, the number of missionaries available was severely limited.

Source: The Saints in France (Ensign: January 1976)
Poland

In 1928, Selbongen (renamed Zelwagi after the Second World War) became the home of a Latter-day Saint meetinghouse and congregation. Selbongen was ceded to Poland from Germany after World War II. Elder Ezra Taft Benson, then a Church apostle and later United States secretary of agriculture, visited Zelwagi soon after the war. More than 100 members and friends gathered to sing, share testimonies, pray and listen to his counsel. In 1947, authorities ordered Zelwagi meetings discontinued because Polish was the only language the government allowed to be spoken at public meetings. (As former citizens of Germany, the members conducted their meetings in German).

Source 2:

After the war, many of the previous LDS branches in eastern Germany now resided in the realigned Poland, meaning many members soon left or were forced out. The branch in Selbongen – renamed Zelwagi by the government — earned a post-war visit by Elder Ezra Taft Benson in his nearly yearlong European welfare mission in 1946. More than 100 members and friends gathered for a quickly convened meeting.

Source: Poland - LDS Facts and Statistics
Source: LDS Church in Poland has had long, hard journey
Russia
I haven't found anything for that time period in Russia, but if you are interested about general history, there are quite a few articles. Mormons in Russia, The LDS Church in Russia

With the Bolshevik-led October 1917 revolution, atheism became the official religion and the Bible was not widely read or available. The country was closed to all Christianity, which included The Church of Jesus Christ of Latter-day Saints.

The church wasn't officially recognized until 1991.