Burns often identified sources in his letters, but doesn't seem to have thought a wider public needed to know.

Sharpe's largest single oral source was his childhood nurse. On paper he used a ragbag of stuff like the notebooks of Robert Mylne and the tunebooks of rural instrumentalists. Laing was more extreme, dumpster-diving outside lawyers' offices for historical documents.

For song texts, I suspect the slip songs and chapbooks were more reliable than the artier and more prestigious publications - after all they were expecting to sell the stuff back to the same class of people who created it. You see that very clearly with tunes - fiddlers' tunebooks are way more usable than any of the posh copperplate-engraved coffee-table collections.