When you boot from the install media, wifi works.
So you could also reboot on the install media (cdrom, usb-key, or usb-hdd..)
chroot into your installed system and then
sudo apt install bcmwl-kernel-source. After that you can reboot, unplug install media,
bcmwl-kernel-source will still be installed on your system, wifi will work normally.
To chroot, boot from the install media, open a terminal, mount your installed system on
$ sudo mkdir /media/target && sudo mount /dev/sda2 /media/target
$ sudo mount ‐‐bind /dev /media/target/dev
$ sudo mount ‐‐bind /proc /media/target/proc
$ sudo mount ‐‐bind /sys /media/target/sys
$ sudo chroot /media/target
$ sudo apt install bcmwl-kernel-source