Shortcutting: the grotto below the crack within the pocket regio is home to a magical cave salamander who has the power of invisibility. If physically trapped or held, he will drop his tail and run away; the tail contains a pawn of Perdo vis and takes a few months to grow back. The salamander's dung fertilizes the fungus in the cave, especially in a particular corner, where the fungus too is rendered invisible; it can be harvested for Imaginem vis.
Mechanically, the covenant acquires a vis source: 3 pawns Perdo and 4 pawns Imaginem per year, every winter for simplicity (in 1223 let's say we get 1 pawn Perdo and 2 pawns Imaginem only). The salamander probably has a poisonous bite (not fatal) but is not built for combat, so reasonable precautions should allow the gathering of the vis; Second Sight is also helpful. One wrinkle is that the entrance to the pocket regio is smack dab in the middle of the plateau on which the Tribunal city is being built. Maybe the Administration building can surround it...?
(Jonathan, here's your chance to confirm or question this allocation of Tranquillina's build points (see also this post unrelated to the vis source). Anyone can offer suggestions about tweaking the vis source if they want.)