I didn't see Panther on Apple's website for sale, so you might want to check on eBay.com. Panther comes on CD-ROMs so you shouldn't have a problem installing it. An iMac G4 should run it perfectly fine. As to upgrading to 10.3.9, it's just a simple download after installing 10.3. After booting into 10.3.0 a dialog should come up asking you to upgrade your software and you'll see something like "Mac OS X 10.3.9 Combo Update".
Why not skip upgrading to Panther and go straight to Tiger 10.4. I'm pretty sure you can order Tiger on CD-ROMs. Might wanna check into that... Tiger is a really good OS.