The merchant that sells firing sheets is located
at the first "Inn" along the path (the second set
of buildings). If you face the front of the inn
he is in the hut to the left of the inn. Often
times he will be on the roof or wandering nearby
(yet another bug, err.. "feature" from Verant).
If he is in this state you are basically out of
luck since since he won't sell. You will have to
wait until a server reset which will put him back
in his hut or try someplace else like Kaladim or
Rivervale. His last name is Clayfinger.