There is not a lot of notated overtone music out there, particularly for choirs. And even less so when it comes to polyphonic singing.
In the earliest days of the appearance of composed western overtone music, there was Folke Rabe and Stockhausen.
Later on, Sarah Hopkins (Australia) created 'Past Life Melodies', which became internationally known and performed by many choirs around the world. Many such choirs were not overtone choirs, per se, but her piece did not require a high level of skill in the overtone parts.
Stuart Hinds (US) has recently published overtone choir music. His first set of 3 compositions was published in Germany by Hofmeister in 2008, and now he has a larger collection (15) coming out. It's really great to see this getting more attention, and I wish him success in getting this out there in the world. Hopefully, with more written scores available, it will encourage regular choirs to evolve into overtone choirs.
There is also:
Baird Hersey (US)
Linda Alexandersson (Sweden)
I am also trying to get my own compositions out there, and the best way I can think of is to widely offer the scores for free. For example, I don't want to waste any time in having 'The Dawning of the New World' performed, given the sentiment involved. We could use a new world as quickly as possible.
So at present, I offer 8 of my scores (7 originals, 1 arrangement) for free download. If anyone wants mp3s using the Sibelius player (choral library) let me know and I'll send the desired ones.
Kiva's overtone choir scores