Software components in Debian are installed using .deb files.
deb (file format)
Using search in Debian Search package directories
The package you need is wpasupplicant and the jessie page for it is Package: wpasupplicant (2.3-1+deb8u3) [security]
From uname we know you have an amd64 system, so a possible URL for it linked by that page is as of today:
wpasupplicant package from main update
After downloading the wpasupplicant*deb file in another PC, bringing it over with a pen, and mounting it, you can install it using dpkg with the following command:
sudo dpkg -i wpasupplicant*amd64.deb
Dependencies (from /var/lib/dpkg/status)
Depends: libc6 (>= 2.15), libnl-3-200 (>= 3.2.7), libnl-genl-3-200 (>= 3.2.7), libnl-route-3-200 (>= 3.2.7), libssl1.0.0 (>= 1.0.1), lsb-base
Some of them are probably installed.
Using again https://www.debian.org/distrib/packages#search_packages
http://ftp.us.debian.org/debian/pool/main/g/glibc/libc6_2.19-18+deb8u1_amd64.deb
http://ftp.us.debian.org/debian/pool/main/libn/libnl3/libnl-3-200_3.2.24-2_amd64.deb
http://ftp.us.debian.org/debian/pool/main/libn/libnl3/libnl-genl-3-200_3.2.24-2_amd64.deb
http://ftp.us.debian.org/debian/pool/main/libn/libnl3/libnl-route-3-200_3.2.24-2_amd64.deb
http://security.debian.org/debian-security/pool/updates/main/o/openssl/libssl1.0.0_1.0.1k-3+deb8u2_amd64.deb
http://ftp.us.debian.org/debian/pool/main/l/lsb/lsb-base_4.1+Debian13+nmu1_all.deb
This may bring some other dependencies, which you can find again in the search page.
Nonetheless, if in another working Debian amd64 system, instead of downloading one by one, you can download a package and it´s dependencies:
How to download all dependencies and packages to directory
sudo aptitude clean
sudo aptitude --download-only install <your_package_here>
sudo cp /var/cache/apt/archives/*.deb <your_directory_here>
Note: If this doesn't work with your actual version of Debian, try do this instead:
sudo cd /somedirectory
sudo apt-get download PACKAGE && apt-cache depends -i PACKAGE | awk '{print $2}' | xargs apt-get download
As for the sources.list file, comment for now the line that starts with "deb cdrom:" in /etc/apt/sources.list
I then recommend that after you have Internet connectivity, you put your sources.list as follows:
deb http://httpredir.debian.org/debian jessie main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
And run the commands:
sudo apt-get update
sudo apt-get upgrade