I would like to test detection of a stealth keylogger for Windows (preferably one that works well on Windows 10).
Where can I find either a closed or open source keylogger that does not appear in the Task Manager?
I would like to test ...
Being a Test Manager I'd say you don't need a single product but many products in order to test whether your anti-keylogger works well with any of them and not only with a single one. Other than that, it seems there's no need to find out the most suitable one, since they are only used for the purpose of being detected. Any other features or usability are not important.
I'd like to add one more requirement for this scenario, which is the availability of a trial version, since you potentially don't want to buy all of them, because you don't want to use them actually. All of the following have a trial version available.
The list on keyloggers.com was helpful to find these programs which match your requirements:
works well on Windows 10
The website has tested on Windows 8, so I looked up all of them to see if they are advertised for Windows 10.
a stealth keylogger ... not appear in the Task Manager
This is already a criterion on the website itself, so I analyzed only those which were listed as "Hidden in Task Manager".
Here's the remaining list of products that match your expectation. I am listing the direct links to the vendors, not the highly advertised ones on keyloggers.com: