There is a Mercere Quarter of the covenant, by agreement.
This isn’t actually a full quarter, nor does it mean that their space entirely contiguous. But this does include most of the space of the largest tower of the covenant, as well as a number of smaller buildings within the walls of the outer bailey, and a large house down in the village below. They control two labs in the tower, one that is used by visiting Mercere magi to train and study, and another that is for the extraction and handling of vis.
At any given time there is a miscellaneous junior Mercere magus serving a turn for a year working in the vis lab.
Rodrigo gave House Mercere the Grand Tower for use, keeping his private magical pursuits separate. His son Florenzo uses his lab, and claimed there was no mysteries hidden in there. He never let anyone in there.
It is common knowledge that he has his father’s permission to use the lab, but he does not feel he has his father’s authority to allow others in there. Why does he come to Andorra periodically to use this lab when he is a member of Barcelona? Because of the Aura, but mainly to keep an eye out an make sure that House Mercere does not try to take it. He will leave Barcelona and join Andorra to claim his inheritance if he has to.
But he doesn’t want to, because he doesn’t think Rodrigo is dead. The thing is, though, Rodrigo had quite a bit of Warping, much more than Antonio. Severe or Final Twilight is a very real possibility.
House Mercere can get you what you need, at a very moderate price. They give you the insider rates for vis exchange: 1 to 1 for vf to vf or vt to vt, 2 for 1 to change vf to vt, or 1 for 2 when exchanging vt for vf.