To find the nearest (and thus probably the fastest) mirror, see the list of Ubuntu mirrors.
When downloading files from an Ubuntu mirror using FTP, be sure to download the files in binary mode, not text or automatic mode.
The installation images are located on each Ubuntu mirror in the directory ubuntu/dists/bionic/main/installer-ppc64el/current/images/ — the MANIFEST lists each image and its purpose.
The HWE installation images are located on each Ubuntu mirror in the directory ubuntu/dists/bionic-updates/main/installer-ppc64el/current/images/hwe-netboot/. The image will be available soon after HWE kernel is available and before second point release.