techsmith.com/download/oldversions
If the required version is absent in the dropdown, then you can download it by constructing the download URL manually:
https://download.techsmith.com/camtasiastudio/releases/<version without dots>/camtasia.exe
E.g, to download 24.4.3 use the URL:
https://download.techsmith.com/camtasiastudio/releases/2343/camtasia.exe