This unique ring is being dropped randomly, but there are some locations where it’s way more likely to drop. The best way to farm SoJ is by slaying bosses. The bosses with the best chances to drop it are Andariel (on Nightmare), Diablo (on Normal) and Duriel (on Hell).
The odds are the best for nightmare Andariel, with the chances of around 1 in ~7.5k. Then normal Diablo with around 1 in ~9k and hell Duriel with around 1 in ~11.5k chance to drop the Stone of Jordan. As you can see the chances aren’t really that great.