The latest images are always found at the link, it’s name should be something like “firmware-?.?.?-amd64-netinst.iso”:
http://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/current/amd64/iso-cd/
In case you happen to need an older version of the boot image, you can search for the specific version that you need on this link:
http://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/archive/