Can not find In Design CC 2019 to download

IF/When you do have a download, be sure to copy it to AT LEAST one external device
-since Adobe does sometimes remove old versions from the server
-a usb flash drive or external hard drive, or write to a DVD

I need to Download Adobe InDesign 2019 14.0.3 on my iMac that I just purchased

Keep in mind that CC 2019 is only supported on Sierra, High Sierra and Mojave. 

If yours is a new iMac running Monterey, there's a strong possibility that 2019 either will not run or you may  encounter unexpected behaviour. Fixes may only be available by running the current version.

Adobe only allows you to download the 2 most recent versions. Previous versions are not available to download through the app. You can try this link for the links to older installers: https://prodesigntools.com/adobe-cc-2019-direct-download-links.html Once those links go you will have to choose a newer version.