Check this out – someone is selling already grated soap for making your own soap-based laundry powder:
http://www.soapsgonebuy.com/
They sell Borax in bulk, too. I can’t for certain say that all of their products are non-detergent, but most seem so. (I would be careful to check out the unscented “powder” first by asking them directly, because they don’t say what brand of hard soap they start with or what ingredients are in it. However, they do seem to know the difference between “old-fashioned lye soap” and detergent.)
The site also has a section with laundry powder recipes using grated soap.