There are a couple of things we might call "rare provisioning ingredients."
The purple mats used in food and drink come from hirelings and writ rewards. If you need frost Miriam and Bervez juice, invest in provision hirelings and you'll build up a supply
The game also calls the mats used to make furniture "rare provisioning ingredients," like the mundane runes used for certain plans. These all come from their respective nodes in the world as a rare drop.
There's also a tab in guild stores under crafting...provision...rare ingredients if you want to buy them.