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.