Proxmox's pveam update fetches releases from http://download.proxmox.com/images/aplinfo-pve-8.dat
and https://releases.turnkeylinux.org/pve/aplinfo.dat, which can be changed in /usr/share/perl5/PVE/APLInfo.pm.
The dat files contain the names of images which can you use to get the direct pveam download download URLs,
for example: