On this page of the guide to BioShock Infinite, you will find information about where to find infusions / essences in chapters 6, 7, and 8. Information about each infusion features a description and a picture. You can obtain 4 infusions in chapters 6-8.
Monument Island - behind the desk in a room with the Siphon.
Park Ticketing - inside the ticket room (you can collect this item after you've separated with Elizabeth).
Soldier's Field Greeting Pavilion - on a desk in the security office you can gain access to only by breaking in.
Soldier's Field - inside a secret cellar of the Fellow Traveller bar. The main character can access this area by completing the only side mission of the 8th chapter.