To find the nearest (and thus probably the fastest) mirror, see the list of Debian mirrors.
The installation images are located on each Debian mirror in the directory debian/dists/buster/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/ .