Im not sure what the last version Of Linux mandrake that would run on a 486 but I would assume Mandrake 4.0
However I know where to get older version from 7.2 upwards but not anything lower
Oh though you could use Redhat Linux 4 or 5
To download RedHat Linux
Click Here
However this isn't an iso image this is just all the install files
and anyway Mandrake is similar to Redhat as Mandrake originally started as an enhanced version of RedHat
However you will find if you a version like Mandrake 7.2 and you install only what you need and use something like Blackbox or Icewm as your window Manager in X it could still possibly work fine
Also try search for older versions in a search engine like Yahoo you might still come up with something