Here is a list of Soulvizier Mercenary Contracts, for reference.
Mercenary Contracts are dropped on monsters or chests (I can't say the exact numbers, but you have a fair chance of getting one contract in each act without farming).
Formula upgrades can be bought from merchants.
Normal :
https://i.imgur.com/8eQiwvS.jpgEpic :
https://i.imgur.com/sfllB6X.jpgLegendary :
https://i.imgur.com/fL722RN.jpgNormal - act IEuanthe, Night's Flower [Maenad Archer]
Iaera Rimfrost [Maenad Frost Sorceress]
Scyrna Stonegaze [Gorgon Archer]
Normal - act IIKemzir, Scorching Wind of the Desert [Dune Raider Fire Melee]
Upgrade level 2 for Iaera Rimfrost
(formula upgrade)Normal - act IIISuhrab, Murkfire Priest [Ichthian Mage]
It'tak of the Dark Sun Guard [Tropical Arachnos Physical/Pierce Melee]
Upgrade level 3 for Iaera Rimfrost
(formula upgrade)Normal - AtlantisMoonshard the Axolotl [Salamander Crushing/Cold/Electrical Burn Melee]
Normal - act IVApollonia, The Fallen Goddess [Empusa Sun Sorceress]
Normal - act VEldr Stikla, Stone Melter [Centipede Fire Melee]
Epic - act IMivania Frostshade [Maenad Frost Archer]
(Epic upgrade from Iaera Rimfrost contract + Euanthe, Night's Flower contract + Chill of Tartarus)Aithiopikos of the Wrath Blades [Satyr Melee]
Epic - act IIAmenetem, Guardian Anubite [Jackalman Physical/Vitality Melee]
Epic - act IIIThe Jade Wall [Terracotta Physical Melee]
Epic - AtlantisEnegea the Hazy Storm [Triton Lightning Mage]
Progath, Warpfire Elemental [Elemental Poison/Fire Melee]
Epic - act IVIxion, Champion of Hades [Ether Lord Melee]
Vaextor the Shadow Whisperer [Shadow Stalker]
Epic - act VKholündr, Lightweave Priest [Alfr Crushing/Lightning Mage]
Legendary - act IVanati the Stonebinder [Gorgon Archer]
Euanthe, Blood Flower [Maenad Bleeding/Life Leech Archer]
(Legendary upgrade for Euanthe, Night Flower)Legendary - act IINikaure, Cursed Vizier [Mummy Physical/Vitality Mage]
Legendary - act IIIQinan Meng, Keeper of Duality [Tigerman Elemental Mage]
Legendary - AtlantisOmnaos, Streams of Lanthea [Potamoi Physical/Cold Melee]
Legendary - act IVMoklog the Skewering [Troglodyte Physical Melee]
Legendary - act VLangnidiar, the Ever-Flowering [Wood Colossus Crushing/Sleep Melee]
Future updatesYlva may be converted in a renamed Mercenary instead of some free ally (this concept doesn't make sense).