We use source packages, so just download the correct tar.gz or tar.bz2 package for the version thats in portage. Then when you get home, copy the package to /usr/portage/distfiles and emerge will see it and use it.
One easy way to do it, if you can reboot whatever machine you plan on fetching the files from, would be to use the sabayon livecd/dvd and mount your usb device to /usr/portage/distfiles. Then while on the livecd do emerge -f <package1> <package2> and so on. That will fetch the files for each package, and if you mounted your usb drive like i said, put them on your usb drive.