Yep, ebay and discogs are my go to places. Discogs tend to have better, more accurate condition grading but ebay has the buyer's protection if you get a horrible disc listed as "like new" (as has happened to me numerous times). Stuff that's still in print (they are out there) I usually get from Amazon or ebay. You can find some Japanese SACDs on the Japanese Amazon site (e.g. Deep Purple's Motorhead, Eagle's Hotel California, Jeff Beck's Wired/Rough and Ready/etc.) that are often cheaper than buying from U.S. sellers and fast as they usually ship by DHL and arrive in a couple days. Also, be aware than when searching on discogs you should try searching with the term "multichannel" rather than "SACD" or "DVD" as there are often multiple multichannel versions (U.S. DVD-A, euro DVD-A, SACD, all of the same album) of an album that may not all show up if you search by a format type.
VIDEO: Epson 5040UB | Sony 65X900A | Sony 55X900A | Samsung UN40H5500
MEDIA: Oppo UDP 203 | Oppo BDP 103 | Apple TV | Amazon FireTV | Nintendo Switch
RECEIVER: Yamaha Aventage RX-A820
AUDIO: Monitor Audio Silver 100 (front), Monitor Audio Silver C350 (center), Monitor Audio Shadow 25 (rear), Rythmik L22 (sub)