Since they're usually called "Complete ... Album Collection", a discogs search is quite useful ... but if anyone finds a list, let me know please! It's not completely reliable, so you may want to try a few similar searches (i.e. you need to replace "sony" w/rca to find the Peter Serkin box, though the entry has both RCA and Sony Classical listed as labels):
https://www.discogs.com/search/?q=complete+album+collection+sony&type=all&genre_exact=Classical
add a genre filter depending on if you're looking for Leonard Cohen, Earth Wind & Fire and Judas Priest or Monk, Weather Report and Blakey or Fritz Reiner, George Szell and Glenn Gould (that latest category is the one I'm most into with these ... and running such a search does two things: make me realize how I bought gazillions of discs in the past 15 years, and also hurts with regards to boxes I skipped, such as the Rudolf Serkin or the André Previn).
As for the original question, I use Presto Music (never any issue with my credit card there, and they got lots of money from me) ... and sometimes JPC or evil A as well. They offer plenty of lists (I never use, unless when browsing through offers), including one on Sony:
https://www.prestomusic.com/classical/labels/1333--sony