The answer to your question is: "From the application developer itself!".
Below is the explanation how you can (directly) download Ultimaker Cura from the most reputable source: Ultimaker.com
When clicking the link to download of the Cura application download button from the Ultimaker website you are presented with a pop-up GUI that asks you if you want to share where you are using the program for. When you select the required field and press download, the download starts. If this doesn't work, a direct download approach can be used.
If the regular download method fails, or you do not get the pop-up GUI, you can use the direct link for Ultimaker Cura from the developer's website:
This link can be found at the download directory at the website:
This web directory overview also shows all previous releases and installers for other platforms.
As of version 3.6.0, the download does not appear to be in the list, this link will show an overview: https://ultimaker.com/en/products/ultimaker-cura-software/list
Do note that the latter link will show the pop-up.
As of version 4.1, the web directory overview link will serve a 404.
The Ultimaker Cura Project has a Github, where they offer the current and previous editions under: