http://www.info.apple.com/support/oldersoftwarelist.html#system
7.5.3 is there but its in multiple disks you need to join, so its not an ISO nor bootable. There are also updated to 7.5.5 there.
7.5.3 is there but its in multiple disks you need to join, so its not an ISO nor bootable. There are also updated to 7.5.5 there.