CO2Now.org republishes scientific data.   The site makes important climate data and trends more visible.   CO2Now is not a primary source for data.  

CO2Now features data for the concentration of carbon dioxide in the atmosphere.   The main data source is the National Oceanic and Atmospheric Administration / Earth Systems Research Laboratory in the United States, or NOAA / ESRL.   This is one of two institutions that measure atmospheric CO2 levels at the Mauna Loa Observatory in Hawaii.   The Scripps Institution of Oceanography also has a CO2 monitoring program at the Mauna Loa Observatory.   Scripps started the world's first high-precision monitoring program at the Mauna Loa Observatory.

Climate science data is made available by scientific institutions and journals around the world.  If you are looking for a particular data set, an excellent starting point is the catalogue of data sources at RealClimate.org.  RealClimate is a commentary site on climate science by working climate scientists for the interested public and journalists.