Source: Federal Reserve Bank of St. Louis  

Release: St. Louis Bi-Weekly Reserves and Monetary Base  

Units:  Billions of Dollars, Not Seasonally Adjusted

Frequency:  Biweekly, Ending Wednesday

Updates of this series will be ceased on December 19, 2019. Interested users can construct a proxy of the difference between the monetary base from the H.3 release and currency component of M1 from the H.6 release. The discontinued series plotted on the same graph with the calculated data can be accessed for comparison here.
For more details, see the FRED Announcement.