To obtain Ancient Mana, I take my characters to the edge of the city at about 52.8, 48.4 and then one by one take them into the vineyard area just across the road from there - while wearing the disguise of course. There are some patrols in the back and along the edge, but the rest of the guards in that area (the ones that can detect you're a filthy outlander with a fake moustache) don't move around. So once you learn where they are, you can easily avoid them.

The area has frequent spawns of flowers, wine bottles and chests which do show up on the minimap (you get the ability to track Ancient Mana very early on in the story) and tend to yield between 30 and 135 ancient mana each for me. So while I do have to do this for each character individually (haven't had any luck with using broadcasting on those myself, it's always just one character that gets the item), so far I've been able to get about 400 Ancient Mana per 5 minutes spent.