Music Composers Unite!
For Matt and the smattering of others who've inquired, and anyone else who might be interested, these are a couple of the most comprehensive internet resources I've found for cantus firmi.
Don't disount tracking down some old missals at your local antiquarian bookshops either:
In case you're wondering, does anyone actually use these unwieldly databases? Yes, I derived the Pacem Relinquo Vobis subject from an amalgamation of some of the traditional chant cantus found through the latter site.
If anybody decides to do some species, or compositional setting of a cantus firmus found therein, please post it up.