I find https://cdimage.debian.org/cdimage/archive/ https://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/archive/
useful as these have the Free and non-free (i.e. including proprietary firmware and drivers) installers for older Debian versions. Additionally https://www.debian.org/releases/${CODENAME}/debian-installer/ has media links and errata per distribution. ${CODENAME} can be obtained from https://www.debian.org/releases/ .
