There is a web site called the American Religion Data Archive that has Statistics on the religions in the United States. This information is dated 2000. American Religion Data Archive http://www.thearda.com/arda.asp?Show=Home.
Also there is a national survey on American congregations: Faith Communities Today 2005: http://fact.hartsem.edu/CongGrowth.pdf.
The Census is not a resource for Religion Statistics. The Census web site says: "Public Law 94-521 prohibits us from asking a question on religious affiliation on a mandatory basis; therefore, the Bureau of the Census is not the source for information on religion." http://www.census.gov/prod/www/religion.htm.