On this page of the guide to BioShock Infinite, you will find information about where to find infusions / essences in chapters 17-20. Information about each infusion features a description and a picture. There are 2 infusions to find in chapters 17-20.
The Good Time Club - on the bar's counter, opposite the stage and behind a Dollar Bill vending machine.
Plaza of Zeal - inside the watchmaker's store. You need to complete a side mission to gain access to this item.