One post for each source
On a hill south of the Covenant, in an abandoned mine, there grows a crystal. Sometimes the crystal grows quickly and sometimes slowly. Once a year it can be harvested for Terram vis. Harvesters are careful to not break off the root of the crystal for fear that it might not regrow next year.
(The crystal takes from 2 to 4 seasons to fully grow. A fully grown crystal yields 1 pawn of Terram vis.)
To the south and west of the Covenant, deep in the forest, Emericus finds an ancient oak. Gnarled with age, it has a scar from an old lighting strike on it's north side. No branches grow on that side but one small and twisted branch. If you can find some of the autumn leaves from the branch, muto vis can be extracted. If one finds an acorn from the branch, herbam vis can be extracted. Only one type of vis per year is found.
(Each autumn, roll a simple die. A roll of 1-2 yields nothing, 3-8 yield 1 pawn of Muto, and 9-0 yield 3 pawns of Herbam.)