I suppose this doesn't really help you, but I improvise all of mine (although for the chants we do often they are more/less 'fixed' at this point). It's on my bucket list to write them out one day... but I've never had the need, as it were, although I get requests regularly. The only time I use a written out accompaniment is if there are modal shifts that make harmonizing them unusually difficult. (I'll also note that I do not fret about harmonizing according to modal theory. I do what sounds beautiful to my ear. As one person put it to me recently, "the moment you start 'harmonizing' chant modal theory goes out the window anyway; chant is fundamentally monophonic.") The whole of the NOH is available over at CCW, so you always have a starting point there, but horror-of-horrors: I don't care for those accompaniments at all.
I improvise mine as well. I have a few chant hymns that I've written accompaniments to (e.g., "Adoro Te", "Ubi Caritas"), but I improvise just about all of the chant Mass settings (Latin - namely Masses VIII, XVII and XVIII - and English - ICEL). BMP
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