Source: Board of Governors of the Federal Reserve System (US)  

Release: H.4.1 Factors Affecting Reserve Balances  

Units:  Billions of U.S. Dollars, Not Seasonally Adjusted

Frequency:  Weekly, Ending Wednesday

Feb 1984 - Dec 1990: Annual Statistical Digest, various issues, Table 2.
Jan 1991 to date: Federal Reserve Board, H.4.1.
Reserve balances with Federal Reserve Banks are the difference between "total factors supplying reserve funds" and "total factors, other than reserve balances, absorbing reserve funds." This item includes balances at the Federal Reserve of all depository institutions that are used to satisfy reserve requirements and balances held in excess of balance requirements. It excludes reserves held in the form of cash in bank vaults, and excludes service-related deposits