Accessing Avesta for research
In order to get access to Avesta, one of the oldest sources of ancient Iran for the purpose of research on atheism, you can enter the below website:
Assistance from experts
One of the pioneer research institutions on history of Iran, is the Center for Iranian Studies at University of London. You can get assistance on your research about ancient Iran and Zoroastrian sources, by referring to the "Department of Religions and Philosophies" section, in the below website: