Lovely thread yes!
Here are a few from my current saga set in the Rhine:
The Kobold Mine
Vis: Terram (2 per season)
Deep in the mine, fist-sized lumps emerge almost orgaically straight from the bedrock. They need to be regularly picked soon after they have "hatched" and fallen down. To operate in the mine, the magi have an agreement with the spirit of the mine, the kobold "Split" to deliver a dead body every year - to feed it's children. Failing to do this, he'll take miners or people from the nearby area. Killing the Kobold will ruin the vis-source and the mine.
The Apple Tree
Vis: Muto (3 at end of winter)
The majestic ancient apple tree (of unknown origin) in the covenant produces fantastic apples all winter long (covered in a fine, translucent layer of ice that does not melt) - it's as if the frost has crystallized within the tree itself. The very last winter harvest yields vis.
The Spring
Vis: Vim
On the very first day of spring, the very first drops from a specific melting snow source at "the spring" turn into a silvery substance. One must catch them in the air before they hit the ground.
The Mirror Pond
Vis: Imaginem (varies)
A mirror pond in the deep woods (inhabited by a playful and hungry dryad, Morgen, who wants "playmates" to mate with, then drown and feed it's young from previous encounters) where small fairies dance in the spring mist. The minute fairies are elusive, but they can be caught in bottles with very simple spontaneous magic.
(Quickness + Finesse + die) / 2 can be gathered before they disappear.
The Ermine Family
Vis: Animal (4)
Every summer, a certain ermine family near the covenant always has an albino young. This must be killed in the autumn without damaging the fur, before it attacks the covenant's chickens. Its blood and intestines turns into 4 vis. (Note: one magus took one albino ermine to be it's familiar since it's a magical animal, voiding the vis for that year).
The Weird Hole
A strange and hidden small hole in rocky ground northeast of the tower at the edge of the forest. The hole blows warm air and smells faintly of sulfur. Some kind of orange sludge splutters from it and can once a year be scraped clean and subsequently distilled into Vim vis.
(If it's infernal or not is up to the saga...)
The Waterfalls
Vis: Aquam
A majestic waterfall comes out of the forest and into a beautiful valley where dryads swim and play. The waterfall runs with a thunderous roar and generates a certain amount of foam that gathers near it. This thick foam can be collected (with the permission of the dryads, or they'll try to pull one down). However, behind the waterfall is a opening to the next region for a full series of seven waterfalls (from the classic myth). The opening itself will fade away if the vis is collected (may regenerate for next year?).