I think the "Find Source" button should be avilable all the time!
I give a scenario for what i mean.

I start a download where the Limewire only find 1 source for. Let's say this source is from a modem. Later i decide that this connection is to slow and want to search for more sources (the more a user waits the better are the chanses that more sources pop up!)
As it is now when a download have been initialized then the "find source" button is not available but if it is availabe a user could press this button and when more sources are find the current download will be finished faster!!!