To find the nearest (and thus probably the fastest) mirror, see the list of Debian mirrors.
When downloading files from a Debian mirror, be sure to download the files in binary mode, not text or automatic mode.
The installation images are located on each Debian mirror in the directory debian/dists/sarge/main/installer-powerpc/current/images/ -- the MANIFEST lists each image and its purpose.
For floppy-less installation on NewWorld Macs, it may be most convenient to obtain all the necessary files packaged into one Stuffit archive from http://debian-imac.sourceforge.net/ (separate instructions are included in the archive). Otherwise, obtain the normal installation files listed above. Retrieve the files to an HFS (not HFS+) partition on your system. You will also need the yaboot and yaboot.conf files from the new-powermac or powermac archive folder. However, the newest G4 PowerMacs, and those that work without MacOS 9, need the newest version of yaboot; the one in the archive will not work. Obtain the newest version from http://penguinppc.org/projects/yaboot/.