In addition to Child's 8-volume English and Scottish Ballads (1860), Percy's Reliques of Ancient English Poetry (several editions), Scott's Minstrelsy of the Scottish Border (5th ed., 1821), and Motherwell's Minstrelsy Ancient and Modern (three editions), these books among others are downloadable:

George Ritchie Kinloch, Ancient Scottish Ballads (1827)

Peter Buchan, Ancient Ballads and Songs of the North of Scotland, vol. 2 (1828)

Robert Chambers, The Scottish Ballads (1829)

Robert Chambers, The Scottish Songs, vol. 1 (1829)

Robert Chambers, The Scottish Songs, vol. 2 (1829)

Robert Chambers, Twelve Romantic Scottish Ballads: With the Original Airs, Arranged for the Pianoforte (1844)

Norval Clyne, Ballads from Scottish History (1863)

Allan Ramsay, The Ever Green (1724)

Allan Ramsay, The Ever Green (1761)

Allan Ramsay, The Tea-table Miscellany, 13th ed. (1762)

Allan Ramsay, The Tea-table Miscellany, 17th ed., 2 vols. [in 1] (1788)

James Hogg, The Jacobite Relics of Scotland [1st series] (1819)

James Hogg, The Jacobite Relics of Scotland [2nd series] (1821)