Many organizations do polls that include questions about religion. Here are some good places to start.
THE BARNA RESEARCH GROUP, headed by George Barna, conducts polls about Christians and their beliefs and practices. Polls are searchable.
GALLUP POLLS include many on religion. News releases are available on the Web site when a poll is released, but after that they are only available to paying subscribers.
POLLINGREPORT.COM collects polls from different sources on various topics, including religion.
THE ASSOCIATION OF RELIGION DATA ARCHIVES compiles data on religion.
THE CENTER FOR APPLIED RESEARCH IN THE APOSTOLATE at Georgetown University conducts social scientific research about the Roman Catholic Church.
THE NORTH AMERICAN JEWISH DATA BANK collects social scientific studies of American Jewry.