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?
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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: