To find the nearest (and thus probably the fastest) mirror, see the list of Debian mirrors.
When downloading files from a Debian mirror using FTP, 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/stretch/main/installer-armhf/current/images/ — the MANIFEST lists each image and its purpose.
The installation files for systems supported by the armhf multiplatform kernel (see Section 2.1.4, “Platforms supported by Debian/armhf”) consist of a standard Linux kernel image, a standard Linux initial ramdisk image and a system-specific device-tree blob. The kernel and the initial ramdisk image for tftp-booting can be obtained from .../images/netboot/ and the device-tree blob can be obtained from .../images/device-tree/ . The tar archive for creating a bootable USB stick with the installer can be obtained from .../images/hd-media/ .
U-boot images for various armhf platforms are available at .../images/u-boot/ .