I’m cheap. I’ve got a lot of books on my Kindle that are free because out of copyright, and read stuff on Gutenberg. Every so often I come across something that could be interesting in Ars Magica. I’ll include some details about the source if you want to look it up yourself.
Roland’s Squire: From the Complete Works of Mark Twain, a short story called “General Washington’s Body-Servant.”
Every couple of decades a newspaper would publish a story of: Washington’s body-servant just died, age 95 years, who could remember this and that from the Revolutionary war. Each time the story was published as new, he must have been a couple decades older and was reported to remember further back. “When he died the last time, he distinctly remembered the landing of the pilgrims [...] the body-servant was in the neighborhood of two hundred and sixty years old when he died finally”
To use in Ars: Roland’s squire survived Roncevaux in 778, and hid the sword Durandal. For whatever reason, the squire was blessed by God and has not died of old age. Every twenty or thirty years he fakes his death and goes to some new village or monastery (to prevent rumors of a 400 year old person). But he still retells the story of Roland, so if the magi trace the storyteller they can eventually catch up and learn Durandal’s location.
Ancient Sumerian Vis. Was reading a book about Babylonian Laws, and found out this: Cuneiform on clay tablets was used for all sorts of stuff, including contracts and bills of sale. There are stashes of like a roomful of tablets. Sometimes they were in clay “envelopes”, I guess you bake the tablet, put clay around it, then bake that. In 1220, people didn’t know how to read cuneiform.
To use in Ars: If magi can’t read the tablet, then we can say the tablets were ACTUALLY (in game) early curse tablets. Maybe in your game Roman curse tablets weren’t a thing so the Order has no leadcurseworkers, but a Covenant in the middle east has found these clay tablets that might be an unknown type of magic. Or just a good source of Vis. Perhaps only tablets in envelopes still have magic, but other tablets could still be studied as part of Breakthrough research.