You can find an old version of Flash (and other software) on oldversion.com, though that is not an official site and downloads must be regarded potentially dangerous. Also, if you don't know which version you are looking for (i.e. which one suits you best), it might be a bit of a hassle to find out.
To be honest, I wouldn't even consider installing Flash on an old notebook. I also don't do it on newer computers, because I still have to find a reason for it to be there.