Please see ASF Vertex (https://vertex.daac.asf.alaska.edu/) for SAR data from different missions such as ERS, ALOS, Radarsat, and Sentinel-1.
Sentinel-1 data can also be downloaded from the Copernicus website.
ERS, ENVISAT data can be downloaded from the ESA OTF service (https://spacedata.copernicus.eu/web/cscda/data-access/access-to-esa-eo-data). You have to write a short proposal for accessing the data. Raw SAR data can also be requested from this service.
ALOS-2, Cosmoskymed, TerraSAR-X SAR data .. are also available. But these datasets are not free.
I would recommend you to also read SNAP forum and Sentinel-1 manuals from the Copernicus site. The most important ones are TM-19 manuals, in my opinion.