Did they say where they got it? Generally, people record arrangements of the Copper Family set. The song has also been found in oral currency in Scotland, but under another name: Greig-Duncan III has two examples, Braw Black Jug and Aul' Black Jug.

The song was printed on broadsides, generally without music of course; but Chappell (Popular Music of the Olden Time II, 660-661) quotes the tune from a song-sheet, and Baring-Gould prints it with most of the text (English Minstrelsie, VII, 60). Other broadside examples can be seen at  Bodleian Library Broadside Ballads:

[O] good ale thou art my darling

Beside being available on various recordings, the Copper Family set is in Bob's book A Song For Every Season, which is now in print again.  http://www.thecopperfamily.com/