There are some Grocers (not Chefs) in some towns that sell prov mats. Check the larger towns, like Daggerfall or Skywatch for example. Fortunately, they sell the things that are needed for most recipes, like Broth etc. It's not cheap, but you can buy some stuff to use up some excess that you have.

However, prov mats are in Crates, Barrels, Bags and Backpacks. Plump Worms come from Alchemy Plants and some undead drop them too. You can find the worms in containers, but more so as loot drops.