Where can I find the Tahoe OS for Sequoia 15.8?

Confirm that your Mac is compatible with Tahoe. Check this list:

macOS Tahoe 26 is compatible with these computers - Apple Support

If your Mac is not on the list, then it cannot run macOS Tahoe. In that case, macOS 15.8 is the latest version of macOS for you.

The current version of macOS Tahoe is 26.7.  If you Mac is also compatible with macOS 27 Golden Gate, then you will be offered that as an upgrade. MacOS 27 launched just over a week ago, and that is now the current version of macOS so Tahoe won't be the default choice for an upgrade from Sequoia for Macs that can run macOS 27.

See: Update macOS on Mac - Apple Support

If you want to continue running macOS 26 Tahoe and skip the upgrade to Golden Gate for now, then to update to the latest version of Tahoe do the following:

• Open System Settings > General > Software Update.
• Click the (i) next to Other Updates under Also Available.
• Select the update(s) you want, which may include both macOS Tahoe and Safari updates.
• Click Update Now.

If you do not see Tahoe listed there and your Mac can run it, then you can download the full installer via Terminal. Open Terminal (Spotlight search or Applications > Utilities), then paste in the following command and press enter:

 softwareupdate --fetch-full-installer-version 26.7

You can also use one of the other methods listed here:

How to download and install macOS - Apple Support

If you are running macOS Sequoia 15.8 and want to upgrade to macOS Tahoe 26, first confirm that your Mac is supported.

Apple's current compatibility list is here: macOS Tahoe 26 is compatible with these computers. Make sure your exact Mac model appears on the list.

If your Mac is compatible, go to  > System Settings > General > Software Update. If Tahoe is being offered to your Mac, you can download and install it from there.

You can also use Terminal to check whether Apple is making a full macOS 26 installer available for your Mac. Open Terminal and enter:

softwareupdate --list-full-installers

Look for an entry showing macOS Tahoe, Version: 26.x.

If macOS 26 appears, you can download the full installer with:

softwareupdate --fetch-full-installer --full-installer-version 26.x

Replace 26.x with the exact version shown by the previous command. Apple documents this as a supported method for downloading a full macOS installer. (Apple Support)

When the download finishes, the installer should be in your Applications folder as Install macOS Tahoe.app. You can then open it and proceed with the installation, or leave it in Applications for later use. (Apple Support)

If Tahoe does not appear in Software Update and softwareupdate --list-full-installers does not show macOS 26, first recheck your Mac's exact model against Apple's Tahoe compatibility list. If the model is supported, the absence of macOS 26 from the Terminal list is the point at which further troubleshooting is appropriate.