Instructions here:
https://www.debian.org/releases/stable/amd64/ch04s03.en.html#usb-copy-flexible
Substitute your mirror…
Generically:
https://debian.osuosl.org/debian/dists/stable/main/installer-amd64/current/images/hd-media/ http://ftp.debian.org/debian/dists/stable/main/installer-amd64/current/images/hd-media/
Specifically:
https://debian.osuosl.org/debian/dists/buster/main/installer-amd64/current/images/hd-media/ http://ftp.debian.org/debian/dists/buster/main/installer-amd64/current/images/hd-media/