This is a frequently asked question I get this about 5x a week :)
Here are leads:
1) Agri/poultry/farm stores that sell feed ingredients
2) Market areas where there are horses in the area (because it is used to make their grass palatable)
3) Feedmills, they also sell ingredients
4) Sugar centrals
5) Other agri/farm/poultry supply enterprises
6) Ask salesmen of veterinary/feeds. They have a network of connections.
7) Other organic farmers
If you think of other leads, please post :)
Price varies...of course, closer to the source, it is cheaper.