Simple answer is that in general you cannot completely isolate particular instruments from a song. You can try applying narrow EQ cuts on selected frequencies which may help quieten unwanted sounds, although it will also affect the sounds you wish to keep. There will be various VST plugins that will certainly help bring out one element over another, but there's usually still remnants of everything else in the background. There's this via Audacity, but again, it isn't exactly what you would call 'isolating' and it affects the clarity of the vocal immensely
To successfully sample a pure isolated drum break or vocal from a song you either need access to the original multi track recordings, or of course the most common way is to find songs which have isolated drums or vocals as a natural part of the song.
