'Find More Sources' Button
 

I think that a 'Find More Sources' button will be useful because sometimes I am downloading a file from 2 hosts and it happens that they are the slowest among 30 people who have the file. That feature was available in Kazaa.

It's called "repeat search" in LimeWire (the right-click on the search tab). Still, having a button that takes a file name and extension and enters it in the search box is a good idea.