Go-to method for finding obscure versions of songs?

So like many young teens growing up during the heyday of Napster, Limewire, Kazaa, etc, many hours were spent downloading songs at a blistering 56k, and many times, the versions of songs would not always be from the original album. Either way, once those songs were downloaded they were then burned on CDs and listened to over and over.

The rub of it is, you get used to hearing a specific live version or whatever then over time, the CD gets damaged or lost and that particular version seems to have disappeared into the depths of the internet.

I'm thinking of a specific version of All Along the Watchtower by DMB, no not the Red Rocks version. Also a bunch of Acappella that I had labeled as U Penn that I can't seem to locate by Google search.

What's the best way to locate these songs? Preferably legally.