Paul McCartney's bass track, isolated through the magic of Rock Band 3 (Celebrating Band on the Run Track Pack! I've included some Let Me Roll It single labels (it was the B-side to Jet).
You can download most of the Guitar Hero and Rock Band tracks from the same place I did:
http://multitrackdownloads.blogspot.com/2012/03/multitracks-w.html
Once downloaded, you can open the files in Audacity (http://audacity.sourceforge.net/) and mix, remix, and isolate to your heart's content!
Let Me Roll It on Wikipedia: http://en.wikipedia.org/wiki/Let_Me_Roll_It
Check out Paul McCartney's isolated vocal track here: http://www.youtube.com/watch?v=cjUuc-9jUos
lead guitar: http://www.youtube.com/watch?v=K0m0si2Gyvw
studio instrumental: http://www.youtube.com/watch?v=4JaXa3BirG0