The stones and mortar were outside the niche in which Fortunato was chained. They were both concealed under a pile of human bones, as Montresor states.
I busied myself among the pile of bones of which I have before spoken. Throwing them aside, I soon uncovered a quantity of building stone and mortar.
Montresor would have had to mix the mortar in a big trough using a shovel. He could have prepared this mortar ahead of time. It was kept moist because of the water constantly dripping on the pile of bones. Eventually he would probably have to come back to the scene of his crime and get rid of the trough and any leftover mortar. But he did not have to worry about the evidence because he assumed he was above suspicion, since he had made everyone believe that he and Fortunato were the best of friends.
The stones must have been big and heavy. This is suggested when Montresor describes how he put the last stone in place.
I had finished a portion of the last and the eleventh; there remained but a single stone to be fitted and plastered in. I struggled with its weight;
After Montresor puts the last stone in place, he uses up the remaining mortar to "plaster over" the entire outside of the wall. This was intended make it look like the rock walls of the catacombs rather than like the outside of a stone wall. He could have used up all of his remaining mortar for the outside plastering, because he would have wanted the outside to be thick and rough. Then he covered the entire outside of his wall with human bones.
Against the new masonry I re-erected the old rampart of bones.
The many references in the story to human bones is not just to create a mood. The bones are useful to conceal the stones and mortar until needed and then to hide the wall after it has been built.
No doubt Montresor has had little or no experience in masonry, although earlier he tells Fortunato jokingly that he is a mason and shows him his trowel. But the wall is a simple enough project even for an amateur. The little niche in which Fortunato is chained is only about three feet wide and about six or seven feet high. That would mean that the entire area of the wall was less than twenty-one square feet. The wall did not have to be perfect, only sturdy, since it would be entirely covered on the outside by the mortar.