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For OS X versions from Lion up to El Capitan.
The simplest way - if available to you - is to use your Purchase History on the App Store.
If you have ever previously installed any OS X version, it will still be available to you from that list. This should include any public beta versions too.

If none of these methods are available to you, you have two options

For OS X versions from Lion up to El Capitan.

The simplest way - if available to you - is to use your Purchase History on the App Store.
If you have ever previously installed any OS X version, it will still be available to you from that list. This should include any public beta versions too.

#If none of these methods are available to you, you have two options

If you need to create a bootable USB to install from

How do I make a bootable USB OS X installer on a Mac?

Sierra itself has now vanished from the App Store, even for users who did have it in their Purchase History. However, Apple are keeping Sierra fully available, even though High Sierra is out. No Apple ID is required.

• Apple KB - How to download macOS Sierra
• Direct download link from the App Store

If you need to reinstall from Recovery, & were previously running Sierra 10.12.4 or higher, there are now 3 options available.

Sierra itself has now vanished from everybody's Purchase History. However, Apple are keeping Sierra fully available, even though High Sierra is out. No Apple ID is required.

• Apple KB - How to download macOS Sierra
• Sierra - Direct download link from the App Store

If you need to reinstall from Recovery, & were previously running Sierra 10.12.4 or higher, there are now 3 options available.
Hold the appropriate key combo at the boot chimes...

Note: No version of Time Machine can be used to revert to any OS older than that of the latest backup in the Time Machine drive.

This means that to preserve your ability to ever restore to an earlier time & OS, you must swap out your Time Machine drive just before you upgrade the OS. Once it sees the new OS, there's no going back.
You can pick data out manually, but cannot use it as a restore to the older OS.