If you're looking for just lyrics, the Digital Tradition database is a great place to start. If you look at the box in the upper right hand corner saying "DigiTrad Lyrics Search", you can use that to search for words, fragments of lyrics, or by using "@work" or other designations (I think they're in the FAQ, or I believe you can also find them in the 'help' link next to the title. I'll be honest, I don't remember for sure).
There are some references that you may find helpful as well. If you're dealing with American studies, I'd suggest Lomax's "Folk Songs of North America" or Sandburg's "American Song Bag". Both provide lyrics, some music, and a degree of information on the history of the songs.
Again, if you're dealing with American, Lomax's field recordings are also a treasure. As an example, the opening work chant from the "O Brother Where Art Thou" soundtrack is a Lomax field recording of a chain gang - a real one. He did a LOT of recording, and there was a CD release of the Library of Congress archives of his work not too far back.
Hope this at least provides a start. More of the crew should be along shortly.