Where can I download a Thunderbird version for Mac OS 10.11.6 (El Capitan)?

You won't be able to use the latest version since Thunderbird requires at least macOS 10.12+.

Even if you can find a version to download, it probably won't work because it uses browser technology to work. The Internet is constantly changing so old apps based on browser technology quickly become out of date. Even if an old version of Thunderbird still works to get your messages will it even be able to display them? It probably won't work for much longer anyway. You are better off using a browser based IMAP email so you can use a standard web browser to access the email. Only Chromium based web browsers are still supporting macOS 10.11, but I don't expect this to last much longer either.

Here is where you can get older versions of Thunderbird (this is the developer's site for official downloads), but you will need to figure out what the last version is compatible with macOS 10.11 (I know Firefox stopped supporting macOS 10.11 a couple years ago so I assume the same for Thunderbird):

https://ftp.mozilla.org/pub/thunderbird/releases/

If your Mac is 2010+, then you may be able to upgrade to a newer version of macOS where the latest release of Thunderbird is still supported. You can get the exact model of your Mac by clicking the Apple menu and selecting "About This Mac". Then you can use this article to see which versions of macOS are compatible with your Mac:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

If your Mac is able to support a later version of macOS, then you can find links to various macOS installers within this Apple article:

How to create a bootable installer for macOS - Apple Support

Otherwise depending on your workload you can consider using a non-Apple OS such as Linux Mint or using a newer computer.