First off, I wonder if you have seen the mtholyoke.edu link below. It has links to a wealth of primary documents that might well be of help to you.
As to the substance of your paper, you are going to have to decide what you mean when you speak of US attitudes. Are you talking about official foreign policy, about FDR's own opinions, or about the opinions of the people in general? The answer to your question could be very different depending on whose attitudes you are trying to study.
If you are talking about FDR's attitudes (which would probably be the easiest to study and to find sources for), I would say that the German occupation of Czechoslovakia was the main turning point. FDR had for some time been much more anti-Nazi than the average American. He thought Hitler was a danger while most did not. However, he did not think he could really get any policies enacted that would directly oppose Germany. When Germany occupied Czechoslovakia, that changed. At that point, he urged the repeal of the arms embargo and asked Congress to allow the US to sell arms to Britain and France.
Congress did not, at that point, agree. But from then on, FDR took more and more measures to actively help the allies. So, from the point of view of FDR's attitudes, I would say that the German occupation of Czechoslovakia was the turning point.