The Stone of Jordan is a unique ring in Diablo 2: Resurrected which, like all unique items, drops randomly and cannot be found in any specific location. The earliest enemy that can possibly drop it is Diablo at the end of Act IV on Normal difficulty. After that point, it’s possible it could be dropped even by a regular monster but, of course, the higher the rarity of the monster, the higher the probability of dropping this (or any other) unique item.
The monster with the single highest chance of dropping The Stone Of Jordan is Andariel on Nightmare difficulty. She also has a pretty good chance of dropping it on Hell difficulty. Diablo on Normal difficulty has the third-best chance. After that, there’s quite a big drop-off, so if you’re really determined to get The Stone of Jordan, play one (or all) of those three boss fights repeatedly until you get it. If you don’t want to do quite such a repetitive grind, then the general drop chance for The Stone Of Jordan peaks at about halfway through Nightmare difficulty, so repeating Act III on Nightmare difficulty could be a more fun and varied way of farming one.