To get the spell, first you must reach level 90 conjuration. Once you have gotten that, go the the College of Winterhold and talk to the Conjuration Trainer there, Phinis Gestor. He will have a new dialogue option that's something along the lines of "What else can I learn to conjure" He will mention some stuff about permanent bindings and sigil stones, and then give you an Unbound Dremora Conjuration Spell. Go to the roof of the Hall of Attainment and there will be a small spot in the middle. Cast the spell there, defeat dremora, recast, redefeat and repeat until the Dremora submits. After that, he'll just say I'll get the sigil stone and you have to recast the spell (again) and he'll come back with the stone. Take it, talk to Phinis and you'll finish the quest + automatically get Flame Thrall, which is the spell for unlimited Flame Atronach. He'll have in his shop the spell "Dead Thrall", "Frost Thrall" and "Storm Thrall" after that, which resurrect dead bodies summons a frost atronach permanently or a storm atronach permanently respectively. They all cost around 4 thousand gold
Beware though, if you don't have the Master Conjuration Spells for half magicka you probably won't be able to even cast the spells, because they cost around 5-600 magicka without the perk.
Alternatively you can enchant head, chest, necklace and ring apparel for Conjuration in order to reduce the magicka cost.