Where is the link to download iOS 10.3.3 for iPhone

Settings > General > Software Update will download and install the most current software available for an iPhone 5s. That will be the current OS 11.2.6 release, according to the MacTracker listing for that device. There's no supported path available to avoid the upgrade to iOS 11.

There isn't a published download for iOS.

To reinstall iOS on an iPad or iPhone, use iTunes and restore the contents of a backup, or use iCloud and restore a backup from there, or use DFU to reload and start over.

There's no downgrade available from iOS 11 to earlier releases, so you'll have to be using an iOS device that was last supported on iOS 10 to be able to re-download and continue to use iOS 10 and then only for and on that device. Other downloads for devices that are supported with iOS 11 will download the current iOS 11 system.