My main go-to is "Discogs" though the price of SACDs is becoming increasingly eye-watering.
Another online store to check out is "CDJapan".
I find that Japanese pressed SACDs (and Japanese SHM CDs and SACDs) are of superior quality.
FYI I have a Hegel CD player and a Marantz KI-Ruby SACD player. Earlier in my audio journey, I found that the Marantz SACD player was unmistakably superior to my Hegel CD player. However, after connecting my Hegel CD player to a Hugo TT 2 DAC and the M-Scaler AND upgrading my cabling, my Hegel CD player sounds quite a bit better (more body, better bass, increased separation) than my SACD player. It has calmed my hunt for SACDs down substantially.