The short answer
You cannot buy a cauldron directly. It unlocks through the main story quest "Mend it with Magic", which community reports place in winter and tie to your progress with Fiona.
- Progress the main story until "Mend it with Magic" triggers.
- Viktor delivers your mother's broken cauldron to the farm.
- Bring Fiona 2× Mana Essence so she can repair it.
- You learn the cauldron blueprint and your first potion recipe.
Where new recipes come from
After the unlock, additional potion recipes are bought at Sabrina's shop, Webb of Wonders. Recipe prices documented so far run from 2,000 to 6,000 — see the recipes page for the full list of known potions, effects, and ingredients.
What is still unverified
The exact trigger conditions (calendar date vs. Fiona friendship level) are not confirmed in any official source yet. This page will be updated when the requirement is pinned down.