Since software versions update frequently, you can find the specific or the latest stable version at these trusted sources:
Sites like PortableApps.com often host clean, pre-packaged versions of popular utilities for mobile use.
Always scan downloaded .rar files with your antivirus software before opening them to ensure the file is safe and hasn't been tampered with by third parties. RAR 6.10.Build103 APK Download - Softpedia
If you specifically need the 6.03 legacy version, check MajorGeeks or OldVersion.com , which maintain archives of older software builds. Quick Tips for Using WinRAR Portable