I thought I'd describe the vis sources as a kind of journey from the main halls of the covenant up towards Iosephus' sanctum. Think of it as a tour of the grounds!
Behind the covenant's beautiful halls, there is a cliff. There is a narrow, rocky path - almost just a ledge - that winds up the side of the cliff. If you climb that path, you'll wind upwards and upwards until you eventually come to a wider ledge, where there grows a shady grove of mulberry trees. The trees are full of silkworms, weaving.
The discarded silkworm cocoons can be collected and made into silk. The silk can then be re-woven into patterns of magical significance. The re-woven silk patterns are worth 5pts of either Animal, Creo, Muto, or Imaginem vis, or any combination thereof, depending on the pattern chosen.
Stay on the ledge near the mulberry trees. If you carefully shift aside the branches of the trees - taking care not to damage any of the cocoons being spun - you'll discover the entrance to a cave. The cave is narrow at first, but as you follow it deeper into the cliff it widens out into a broad cavern.
This cavern is highly magical. Its ceiling drips with water, forming long stalactites, stalagmites, and pillars. These rock formations grow and change with extraordinary speed - extraordinary in geological terms, that is. Every year there are new formations, and the cavern looks different and must be navigated differently.
Periodically, parts of stalactites will break off and fall to the floor, shattering into strange shapes. Certain of these rock shards contain vis: you can tell which ones they are by the shape. If collected, ground up into a fine powder, and mixed with lime (the mineral, not the citrus fruit!), they are worth 5pts of Terram, Muto, or Vim vis per year.