In some cases, you kinda have to know the names already. I’ll help a bit.
Follow MagnumPhotos, DanWintersPhoto, and _rodnetsmith on insta.
That should be enough to get you started. Magnum is a pseudo-agency run by photographer members. Dan Winters is an editorial photographer. Rodney Smith was an editorial/fine art photographer.
I have no experience with MPB, but I have seen some MPB stories here you might want to look into.
I have been buying from KEH for decades with zero issues.