Finding a legitimate, working link for a macOS 10.3.9 download is significantly harder than finding OS X 10.6 (Snow Leopard). Apple does not provide official support or direct downloads for operating systems this old through the App Store.
For those seeking ISO files pre-made for virtualization, GEEKrar is a great resource. It offers both DMG and ISO versions, often with helpful guides.
It supports a huge range of "New World" PowerPC G3, G4, and G5 Macs.
Do you currently have a with an optical drive to burn discs? 3.9 combo update patch ?
Complete Guide to Downloading and Installing Mac OS X 10.3.9 Panther
Use a modern computer with an optical drive to burn the downloaded ISO or DMG files to physical CD-Rs. For Macs, using a slow burning speed (e.g., 4x or 8x) helps prevent read errors on older optical drives.
PowerPC G3, G4, or G5 (running at a minimum of 233 MHz).
Before initiating a download, verify that your target hardware meets the baseline demands of Mac OS X 10.3 Panther. Hardware Prerequisites
