And to add to Tim Radford's list there's also the fabulous archive of Scottish material (from the School of Scottish Studies mainly) which you can find by typing Kist o Riches into your search engine.
These don't include lyrics, but you listen to the original source singers and can take the words from that.

I'm like many others in probably starting from the position of being impressed by either a live performance or, less often, by a lyric. But I also love to trawl through my many books and quite often come across a text that's sufficiently interesting to make me want to do something with it (even if I have to cobble together a tune of my own).

I probably allow myself quite a bit of leeway, but I'm past the age when I want to replicate someone else's performance (and my voice is probably beyond its capacity to do so!). And, in any case, I've always felt that the song itself should have centre stage rather than the performer, so that's definitely what informs my song choice now.