I'm trying to track down coats of arms for the following people:
But I don't know even where to start. Any ideas, even in general of where to look for fifteenth century French and Italian nobility?
See the references and links at the bottom of the page.
Rely on the sources rather than Wikipedia in this case.
Update: This article on François Guillaume de Castelnau de Clermont-Lodève, Archbishop of Narbonne, appears relevant. He was born in 1480, 50 years after Tristan's death; the central part of the coat of arms may not have changed. But all this is supposition, included because you wanted leads.
Note the galero in the image: