There are many places to find end-of-support/"Abandoned" software.
Abandonware constitutes as mainly:* End of support products *
This means software up to Windows ME is abandoned in it's "End of Life." Windows 2000, XP, etc. are still supported, so they aren't available.
A good resource I vouch for is Abandoned ABW.
It's a good site for everything from Windows 98 to old obscure Apple OSes such as Rhapsody DR2, the precursor from the 90s to OS X on Intel, along with old games. They are also starting to hold a collection of VMWare images for things such as Windows 95.
These OSes are provided due to being way past their extended support dates and are no longer supported by their manufacturer.