The Skin of the Vipermagi is a set of unique armor in Diablo 2: Resurrected, which drops randomly, so there’s nowhere in the game that you’re guaranteed to find it. But there are some places where the probability of Skin of the Vipermagi is higher than it is at others. But it’s never very high, so be prepared to grind the same sections of the game a lot if you want to find it. Unless, of course, you get really lucky.
The earliest possible area in which a Skin of the Vipermagi might drop is in the Crystalline Passage during Act V on Normal difficulty, but your chances of finding one there are worse than 1 in 3 million, so don’t go farming for one there.
The encounter with the single highest chance of dropping a Skin of the Vipermagi is Baal on Normal difficulty. Diablo on Normal difficulty also has a decent chance of dropping one, as do all of the end-of-act bosses on both Nightmare and Hell difficulty.
If you don’t fancy farming bosses, then the Secret Cow Level on Nightmare difficulty is a good option. And there’s a relatively high chance of finding a Skin of the Vipermagi in the Forgotten Tower and the Rogue Monastery (particularly its Jail) on Nightmare difficulty, both of which are in Act I. You might get a Skin of the Viper later in the game on Nightmare, or at any time playing on Hell, but the probability is marginally higher in the locations mentioned above.