You can doownload most/all packages from the main Ubuntu repositories from
http://packages.ubuntu.com. You can search for it from this page, find it from here, and download it from one of these pages:
The problem here is that some packages it needs may to be downloaded as well - you can test that by trying to install it, and seeing if it exits with any errors.
Another way of doing it would be to run
apt-get install --print-uris wpasupplicant on the server, and downloading it with the machine that has internet access, and transferring the deb file(s). This will likely be better as it will list other packages it needs as dependencies.
For example, it might give on a Ubuntu 14.04 64bit machine
Once you have managed to get the needed deb files, you can install it with
dpkg -i /PATH/TO/FILE(S).deb.
1: this will likely only work if the machine has had access to the internet to update the package lists - the
print-uris just tells it to print what was going to download instead of downloading it