Viper School Witcher Gear is an addition that crops up in certain places in the game – the standard weapons at the very beginning of the base game, and the armour and venomous weapons at specific points during Hearts of Stone.
At the beginning of the game you can find the Serpentine (Viper) Steel Sword diagram at Amavet Fortress Ruins in White Orchard (west of the Ransacked Village), which starts the quest Scavenger Hunt: Viper School Gear. Note that this ruin is difficult to access initially due to the cliffs surrounding it. What you need to do is wait until after the first encounter with the Griffin, then access the nearby woods via the beach, then make your way north until you find some red brick ruins. Here there’s a fallen red brick column that allows Geralt to climb up to the ruins themselves. Deal with the bandits up there, then check the chest in the main part of the ruin. You’ll find the Serpentine Steel Sword diagram in the chest.
The Serpentine (Viper) Silver Sword diagram can be found to the north west, in the crypt at White Orchard Cemetery. There’s a Wraith in the graveyard which will attack you before disappearing into the crypt below. Blast the door with Aard and then go down and finish the Wraith off. The silver sword diagram is found on the skeletal remains of Kolgrim, near a recess in the crypt.
These two swords are useful early in the game and can be crafted at level 1 & 2 (with the right materials), but after that you’ll hear nothing else about Viper School Gear until you reach level 34 and do the Hearts of Stone quest “Open Sesame!“
To get the Viper School armour, boots, trousers and gauntlets crafting diagrams you must buy them from Countess Mignole at the auction, during the quest “Open Sesame!“. This being a Hearts of Stone quest the recommended level is 34. Note that it’s easy to miss these if you skip certain events, so be careful.
To get the Viper Venomous Steel Sword… The diagram can only be obtained during the quest “Open Sesame!” while inside the vault. It is located inside a chest near Maximilian Borsodi’s House. Make sure you take it before leaving the vault as you can’t return later.
To get the Viper Venomous Silver Sword… This sword doesn’t have a crafting diagram associated with it – you find it ready-made. It can only be obtained during the quest “Whatsoever a Man Soweth…” by challenging Master Mirror. The sword is found protruding from a stone in the dark and sinister Gaunter’s World. Again: make sure you find it and take it before leaving as you can’t return later.