This is the only safe and official way to get the version of iTunes compatible with your specific OS build.
Apple maintains a legacy portal for iTunes downloads. itunes macos 10.13.99 download
First, a quick clarification. Apple’s versioning for High Sierra ended at 10.13.6. If you are seeing references to 10.13.99, it is likely a placeholder reference or a typo for the "end of the line" for macOS 10.13. This is the only safe and official way
If you are running High Sierra, you are on the last major macOS version that features the standalone iTunes app. This is significant because, in this version, iTunes still handles: Apple maintains a legacy portal for iTunes downloads