Slitherine

Hi again, a question again : I got the decision to favor "Emporiums" (it costs a lot !), and a turn message says "Rome can build Naphta Emporium".
But I can't find it to build anywhere, and don't even get what these emporiums are for ?

they are level III commerce buildings, basically they generate goods that are exceptionally hard to find. I think some are great value (Myrhh etc), others a bit mheh for the cost,