The U.S. Senate website, Senate.gov, lists the number of senators by party for every Congress in U.S. history. As of August 2015, there are 54 Republican senators, 44 Democratic senators and two Independent senators in the current 114th Congress, which runs from January 2015 to January 2017.
The Senate of the 114th Congress is the first Senate since the 109th Congress, from 2005-2007, to have a Republican majority. The Senate website also provides a full list senators, which allows visitors to sort by name, state or political party. It also provides contact information for these senators.
The House of Representatives website provides a list of representatives by party affiliation as well.