I've seen on some of the ESO furnishing websites that certain antiquities can be purchased from the achievement furnisher once you have found the lead and excavated the antiquity a number of times and once the codex has been completely unlocked. Yet, I don't appear to have this option. I have found, for example, the lead for the moons-blessed ceremonial pool, as well as the vampire stained glass windows, which I have excavated multiple times yet when I go to the appropriate achievement furnisher it's not listed even though the codex has been completely unlocked for both items.