These vis sources were created for a game held on Bear Island (See Mythic Locations), but could be of interest for a game in Constantinople or Asia Minor.
Lake Manyas hunting ground:
Vis: Animal
Real location: Lake Kuş - Wikipedia
Lake Manyas forms a remarkable breeding ground for migratory bird species. Every summer, regional birds of virtue come to reproduce at this site which has a Magic Aura 3 aligned to Fertility. A hunter who is familiar with Magical Creatures can harvest vis on an annual basis during the season.
Roll Per + Hunt + Lake Manyas area Lore + appropriate magic skill*
*Appropriate magic skill includes one of: Magic Lore, Magic Sensitivity, Comprehend Magic and any other skills the GM deems appropriate to hunting magical creatures. Alternatively, a bonus could be given for the magnitude of the highest appropriate enchantment.
Vis yield: 1 vis on a roll of 9, 1 additional vis per 3 above target.
Regularly employing several hunters may lead to overhunting and damage the local aura. GM should feel free to limit the overall yield of the site. Additionally, the site may become a location of semi-regular stories with mythical animals coming to breed, or disappearing hunters (when a grog or companion is used to harvest the site) which may need to be rescued. From time to time, griffins might be sighted in the area.
Temple of Flames:
Vis: Ignem
Real location: Dascylium (Ergili) - Livius
A ruined magian temple nearby the Lake which has a regio with a Magical Aura of 5, which contains an ancient fire altar, stele, and depictions of ritual sacrifice for a bull. Once per year, if the altar is used to replicate the ancient sacrifice made by the Magians, by burning a bull until cinders remain, the cinders and ashes from the consumed Bull contain 2 Ignem vis. If the site is investigated with high level Intellego Terram spells, the GM should feel free to include hooks to magian magics that may help a character seeking lost secrets.
Marmara Island marble quarry:
Vis: Imaginem
Real location: Marmara Island - Wikipedia
In 1220, the marble quarries of the island have all but stopped, with the chaos created by the Fall of Constantinople, there hasn't been any demande for the material which was used ubiquitously in Ancient Rome. Enterprising magi seeking a new income source might buy the site and restart the business, which covers 40 square kilometers and has a low-key magic aura of 1, except where the dominion aura overwhelms the site's aura in its fringes, after hearing the reputation of the island as the source of Prokonnesian Marble which was used to build so many ancient monuments. The marble, incidentally, contains imaginem vis, and can produce 4 pawns per year if production is resumed. This is actually dedicated vis if used for enchantments without removing the vis from the marble. It has the strange property that, when used to construct monuments and statues, the vis will release itself to empower the crafter's skill and increase the overall artistic value of the craft (treat as a bonus to relevant Craft checks due to an imaginem effect allowing the crafter to visualize his ideal form while crafting. This effects lasts throughout the sculpting / construction, and may incidentally warp the person from prolonged exposure).