You use Spirit Potions when you go to a blue marker. Complete the quest "The Goblin Girl" by fucking Shakala 3 times and visiting the dead tree in the Goblin Forest at a later time. Visit the dead tree again at least 4 days after impregnating her and she takes you to her camp. Talk to Makith once there, bring her the stuff she asks for, and she teaches you how to make Spirit Potions.
Or, complete the quest "The Witch" and be sure to go see her again at least one day later. She teaches you how to make potions. Don't remember if Spirit Potions is one of the recipes she teaches you.
Potions are made at alchemy stations. Makith has one next to where she stands in the goblin camp, and there's another at the Witch's place. You can buy an alchemy desk & install it the rental house once the Witch teaches you how to make potions. There are several other places that either have alchemy stations already or you can buy them when upgrading a place, but those require completing other quests later in the game.
Normally you need to do one or both of those to obtain Spirit Potions. Did you use the key from Freddie's Corpse to get your first potions? No worries if you did. But if you instead edited them (and other stuff) into your game you might have unintentionally messed up some other stuff in the game...
