Here is what I would do to find working download links if their official vendor no longer provides them. The idea is to use Archive.org and see if they have cached those links and if so, you could download from Archive.org and be reasonably sure it is safe to run. So here is the workflow:

1. Find out the exact download link URL by searching web

Once you know the specific older version of the software you are looking for (like Skype 7.5), use google with keywords like "Skype 7.5 download link" or so. You should be able to figure out an exact link like it may be http://download.skype.com/msi/SkypeSetup7.5.0.102.msi in case of Skype 7.5. If that link works - just download your software from there, and you are done. However, quite often the problem is that this link is no longer working - vendor has changed their site structure, decided to block older downloads, etc, etc

2. Use the following URL formula to see if that link's CACHE is available in Archive.org

URL Formula:

http://web.archive.org/web/form-submit.jsp?type=replay&url=.....YOUR-LINK.....

so using the Skype 7.5 link above, the Archive.org url would become this:

http://web.archive.org/web/form-submit.jsp?type=replay&url=http://download.skype.com/msi/SkypeSetup_7.5.0.102.msi

3. Alternatively, you may try to go to Archive.org and MANUALLY search for the cached copy of your link:

1. Navigate to https://web.archive.org and paste your link to the search field
2. Click "Search" button or "Browse History" button
3. See if there will be any "circles" showcasing the dates on which the Archive crawler has made a cached copy of your link. Then pick some date and copy the cached link and see if you could download your desired software package from that link - here is an example of such link: https://web.archive.org/web/20150603121557/http://download.skype.com/msi/SkypeSetup_7.5.0.102.msi

4. References

Some examples of this method in practical use to:

• successfully find Skype 7.5 installation, and also to
• download older Android SDK with Eclipse ADT plugin bundle
• download older Mac OS 9 software
• older RISC OS Python binaries
• some older Microsoft Office 2007 editions
• etc