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Hi Kelly! The Center for Legislative Archives holds Senate lobbying disclosure reports back to 1949. They are paper records until 1983 and then they are on microfilm with corresponding indexes until 1997. You are welcome to visit the National Archives in Washington, D.C. to view these records in our research room.
There are online sources for lobbying reports from more recent years. For reports since 1998, the Center for Responsive Politics maintains a Lobbying Database. Also, the Senate maintains a Lobbying Disclosure Act Database for reports since 1999.
You may also be interested to know that the Federal Election Commission has made campaign finance data available online back to the late 1970s.