How do you use old versions?

So this situation has arisen a few times recently on this list- someone wants to use the latest and greatest but one or more regressions have accumulated and the only fully working version is unsupported old version X.YZ. Ideally those regressions would be solved and we'd all be happily on the latest but that's not necessarily practical or workable in the very short term. It seems like one workaround is for us to plop down on that old working version, but of course the packages have all been removed from the archives. So my question to you is, how would I, for example, install emacs or xfce4 on version 4.0 today? I'm guessing the answer will be cross-compile it on a PC but I'm hoping there's another way.... maybe a mirror of the package archives I'm overlooking? :-)