Source: Federal Reserve Bank of St. Louis  

Release: St. Louis Monthly Reserves and Monetary Base  

Units:  Billions of Dollars, Seasonally Adjusted

Frequency:  Monthly

Updates of this series will be ceased on December 20, 2019. There is no replacement to this seasonally adjusted series. Interested users can construct a proxy of not seasonally adjusted data as the difference between 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.