On this page of the guide to BioShock Infinite, you will find information about where to find infusions / essences in chapters 21-26. Information about each infusion features a description and a picture. You can obtain 4 infusions in chapters 21-26.
Shantytown - in an alley to the left of the elevator.
The Bull House - in a chest in an office on the first floor (optional mission).
The Bull House - inside an armory on the first floor.
Office of Jeremiah Fink - inside a room to the left, opposite Fink's dead body.