Source: U.S. Census Bureau  

Release: Advance Monthly Sales for Retail and Food Services  

Units:  Millions of Dollars, Not Seasonally Adjusted

Frequency:  Monthly

Notes:

The U.S. Census Bureau maintained time series of monthly sales and end-of-month inventory estimates of retailers based on the Standard Industrial Classification (SIC) system. The SIC-based time series were discontinued as the North American Industry Classification System (NAICS) time series were newly introduced.
https://census.gov/retail/mrts/mrtshist.html