I may have misread your original post. Are you asking about the source code for the kernel? I read it that you were looking for the source code for the Cisco driver, sorry.
I'm not on my Slackware distro at the moment, but I think you could grab the source code with this command, if you have a repository defined in /etc/slackpkg/slackpkg.conf:
slackpkg search kernel
slackpkg install kernel-source
I haven't built a kernel in years, so I can't help you with that part, but it shouldn't be too difficult, as long as you include everything that you DO