You have to get the files from somewhere. If those you have are the precise ones the port is looking for, you can place those in /usr/ports/distfiles and go from there. There usually are FreeBSD-specific patches that you also have to get.
Fluxbox might be on one of the installation CDs; if so you can install it from there using sysinstall.
Practically, though, you do need an internet connection. You won't be able to do any updating, bug fixing or installing applications. Well, not easily.
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