https://scihub.copernicus.eu/dhus/#/home
https://earth.esa.int/web/guest/eoli
on-demand processing and delivery of SLCs via EOLI software ("ESA's Link to Earth Observation")
https://centaurus.caf.dlr.de:8443
https://auig2.jaxa.jp/ips/home?language=en_US
https://neodf.nrcan.gc.ca/neodf_cat3
Unfortunately most SAR data is not freely available. Some subsets of data over particular regions are hosted on mirrored archives below.
NASA DAAC hosting entire S1 archive, as well as some ALOS, ERS, Envisat, mostly over north america
Select regions around the world with open-access for select SAR systems