I may have found some vis resources, but they will require some investigation. First, during my stay at Laach Abbey, I wandered around the lake there on several occasions. In late fall, I discovered three perfectly round pumice stones on the south eastern shore. A quick spell revealed that the stones contained Terram vis. I have not been able to return to the lake to look for more stones, but I do have my notes on the discovery, which I submit to the library. Aedituus pulls a parchment from his sleeve, and lays it on the table. I have also included in the notes some details about the ghosts in the area. I studied them at intervals during the construction process, and found that their manifestation changed according to the season and moon phase. Last year, during the dark of the moon in summer, I was able to collect Mentem vis from shreds the ghosts left snagged on the bushes as they passed. Unfortunately, the shreds and vis evaporated when exposed to sunlight, and I have not seen the remains since then. Perhaps, according to their cycle, the ghosts are only strong enough to leave the shreds in the summer.
I also have some details about another source which may be nearby, but have not yet found the source itself. I will let you know when I have either discovered it or have reached a dead-end in the researches.
Finally, I found a letter from a Hermit tucked into a book I was copying at Laach Abbey. The letter contained mundane news, but also seemed to indicate the existence of a book on ghosts. This would not have been remarkable, except that the Hermit wrote a description of the man, who having fallen to the predations of robbers on the road and died at the hermitage, left the book and some other belongings. The Hermit's description included indications that the man may have been a magus. I should like to travel to this Hermtiage later this summer to investigate.