It was never surprising to me Stokes had the Trask silhouette because it's been established that Stokes is a collector of objects that relate to Ben Stokes (Trask is mentioned in Ben's diary) and the possibility that witchcraft was practiced at Collinwood in 1795/96 (Trask being the witch hunter).
As for how the silhouette was made, well, that's an entirely different story. But I suppose there could be any number of explanations. And, of course, Stokes could have come across it in any number of ways - including, perhaps, even while on a trip to Salem to research Trask.
Though for the purposes of the storyline, regardless of how it was made or how Stokes acquired it, the thing that's most important is that Stokes is currently in possession of it.