Ideas for vis sources:
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About half a day away from the covenant, along the Sorne river and a bit south of Underswyler if a small lake called Lac Vert. This might make a great place for a faerie aura and vis source.
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Under the town of Laimunt, there is a Gallo-Roman mausoleum which could be a source of Corpus.
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About a day's travel west of the covenant is a mountaintop called La Roche-d'Or because the light of the setting sun reflect on the rocks. A small statue of a Gaulish tri-horned bull was found there, so there could be a regio with an actual tri-horned bull there, who guards a vis source.
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Something in Rangiers' Pass could make travelling it dangerous to mundanes, but give us a source of vis.
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An old iron mine near Biestinge.
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Ghosts on the Moor - In a swampy moor there live any number of nasty, biting insects which apparently serve no purpose except to annoy travelers. The area is typically avoided by honest folk and for the same reason is something of a haven for outlaws, criminals and bandits who hide there from the local sheriff. The moor is said by some to be haunted by the spirits of those cutthroats who have met their end within, and their ghosts can occasionally be seen as brief, flickering lights out on the moor. Some say that their ethereal voices beckon travelers to join them, and they often do, for the moor is dangerous ground, with many pitfalls and some areas of quicksand. It may be that the outlaws who dwell here encourage such tales, to keep superstitious folk away from their lairs.
Magi who carefully investigate the area will find that the flickering lights on the moor are in fact a unique breed of fireflies, whose glow is an eerie blue. Each such strange firefly contains a pawn of Mentem vis, but there are but a few of these insects in the air on any given summer night. They live only for about a week. -
The Chokehold - There's a very tight passage in the mountains, that is generally avoided by people. Merchants avoid it mostly because it's very narrow, even a horse or donkey would be hard pressed to pass some of it's twists and turns. Others shy from it, because they say it gives an eerie feeling. Every Spring, a couple of loose rocks drop, and each contains Rego Vis.
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The Lover's Tears - There is a very old stone statue of an unknown woman. Although the features are very worn by the passage of time, her eyes remain uneroded. Most people believe it is a representation of the Virgin Mary, and a small altar has been raised around it. But some villagers of the zone talks about an old story about two lovers who were cursed to be separated forever, turned into stone.
Only God knows where the other statue is located, if it exists, but it is said that each year, on the night of the anniversary of its curse, tears fall from the statue's eyes.
Those tears contain vis.
The magi must discover which night it happens, and prevent any annoying rain from diluting the tears (it remains open the possibility of another statue existing, and even what happens if somebody reunites the lovers again). -
The Pierced Stone - in Courgenay, about 10 miles away from Delemont, one will find a large pierced stone. At the dusk, standing at the south face of the rock and looking north through the hole reveals an idyllic vision (a look into a faerie regio). People can cross and enter the regio if they manage to squeeze through, but the village people know better than to do that.
If one leaves an offer on the hole on the night of the autumn equinox (when summer begins to fade into winter), the following day he will find a branch with sweet, red berries. Each berry carries a pawn of faerie-tainted intellego vis. -
Following the Birse river gorge south through the mountains towards Moutier-Grandval, there is a bridge crossing the river. It is a local custom that travellers should drop a silver penny into the river while crossing the bridge to invoke luck and ward oneself against spirits. Sometimes, a penny can be found stuck between the rocks of the rapids. Such a penny can become infused by Aquam vis after a while.
To make things interesting, I like to have at least one vis source be clearly located in each of the three Tribunals. These would be the Contested Resources hook. They don't have to be our most critical sources, but their worth (either the Art or the yield) should be enough that losing one or more of them would hurt. Otherwise that's not a real hook. 
I will make this post a wiki, so everyone can add their suggestions. Then we can discuss them in the Covenant Discussions topic.