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-arm/current/images/ -- the MANIFEST lists each image and its purpose.
The RiscPC installer is booted initially from RISC OS. All the necessary files are provided in one Zip archive, .../current/riscpc/dinstall.zip. Download this file onto the RISC OS machine, copy the linloader.!Boot components into place, and run !dInstall.
The easiest way to boot a NetWinder is over the network, using the supplied TFTP image .../current/netwinder/tftpboot.img .
The only supported boot method for CATS is to use the combined image .../current/cats/tftpboot.img . This can be loaded from any device accessible to the Cyclone bootloader.