This applies to: Debian Linux and derivatives such as Ubuntu Linux.
First of all, you should read what Adam Trickett writes about this. Note that a suggestion he makes is to use apt-file which (at least in Lucid) seems a little buggy. His instructions may also be incomplete (see my comment). I will look into the possible bugs soon I hope.
Once you locate what package to install, one possible route would be to download it (e.g. from packages.ubuntu.com or debian.org) and then just extract its contents with:
dpkg --extract $DEB $DIR
Note that —unpack not only extracts the files, but does other things such as installing them in your system.
If you want a more "managed" installation you can of course try:
dpkg --force-architecture --install $DEB
although this may be inelegant or problematic in practice since would be preferable to have it installed in a /lib32 directory etc. So, for those files not provided by ia32-libs (see Adam's page), consider extracting them manually per above, then placing them in an isolated directory, then symlinking to them from the appropriate directory.
