Here's the song list. Hold on to your hat....
Cân Yr Hen Weddeles
Hob y Deri Dando
All Thro' The Night
Marwnad yr Ehedydd
Land of Our Fathers (Welsh Anthem)
I think that covers it, though I don't know how helpful it is. Most of the music, and much like it, can be found on any decent compilation of Welsh folk choirs. It is a very niche market, but it's there. The culture of Welsh choirs is quite an institution.
Check out these choirs - some of them have Music and CDs for sale on their websites:
If you're interested in purchasing some of this music I suggest Amazon.com
For a sampler try The World of Wales in Song. Amazon has online downloads or you can purchase the CD
The Huntington Theatre Company's production of The Corn is Green, by Emlyn Williams, played January 9 through February 8, 2009 at the Huntington's mainstage, the B.U. Theatre, 264 Huntington Ave, Boston, MA