How can I install my network driver on Debian from a USB media?

Here's what worked for me:

1. In the install screen at the "detect network hardware" step, it will identify the driver it needs. e.g. iwlwifi-7260-17.ucode in my case. Leave the install at that screen.
2. On another computer, search the Internet to figure out what package that driver resides in.
3. Download the .deb file:
   • If you do not have the appropriate non-free repository for the desired package in your /etc/apt/sources.list, you will have to find and download the .deb file without using apt-get.
   • Or, if you have the correct repository in sources (and it's a similar flavor of Linux), download the package you need using apt-get download [package-name].
4. Place the .deb file you just downloaded on the USB media. Be sure to put it in a subdirectory named firmware.
5. Eject the USB drive. :-)
6. Back on the being-installed computer, pop in the USB drive and hit "Continue."
7. It should find the driver and install it correctly.

Theoretically, it's possible to include the firmware on a separate device (like a USB key) during installation, but I've tried that about a dozen times on at least 5 different laptops across the last 3 Debian releases and never gotten it to work. Instead, I usually just use the unofficial non-free net install image.

If you know ahead of time what non-free firmware you'll need, you could download the .deb from packages.debian.org before installation, copy it to some external media, and dpkg --install it after you finish installation.