There are several collections of Scottish Traditional songs available. This is a selection of recommended collections. (Links checked 11 September 2015)
The first SC&T Song book "Songs for Singing. Aberdeen City and Shire" Published 2007. A few copies still available from SC&T - price £5.
Scotland's Songs. More than 130 songs and tunes, in Scots and Gaelic, for schools in particular.
Greig-Duncan Folk Song Collection. This is an 8-volume collection of songs from North-East Scotland, collected between 1902 and 1914. Some of the volumes are now out of print. The collection can be consulted in libraries.
In 2009 a book was published "Songs from North-East Scotland: a Selection for Performers from "The Greig-Duncan Folk Song Collection". There is a web site with the 150 tunes (in midi-files) for the songs selected. As part of her Residency at Celtic and Scottish Studies in the University of Edinburgh, Frieda Morrison has prepared a selection of singers' performances of songs from the "Greig-Duncan Folk Song Collection". These are available in the YouTube channel CeltScotVideos
Tobar an Dualchais - Kist o Riches. Recordings of folklore, songs and music, local history and other data gathered from the 1930s onwards.
The Child Ballads are 305 traditional ballads from England and Scotland and their American variants, collected by Francis James Child during the second half of the 19th century. Their lyrics and Child's studies of them were published as The English and Scottish Popular Ballads. More information is available on the web. One collection of the ballads can be found here.
Other resources worth searching are the Song Collectors Collective and the School of Scottish Studies