A trawl of any bookshop charity or a real one if you can find one can be very fruitful
The other scource is your local reference library or Record Office.
Hertfordshire has some cracking songs and Birmingham Ref has an amazing trove
The Baring Gould collection can and does yield some real gems if that is you can be bothered to read the old mans 'orrible writing
Finding new songs(to you that is)can be a wonderful journey
Good Hunting
Old Nic