The NRCS is not allowed to make referances on builders, but they maintain a list and often quietly suggest who they would personally feel comfortable hiring. My local NRCS has their own supply of clay that builders buy from and I am sure your office knows where to at least get some at. Another option would be to get a load of bentonite delivered to disc into your soil....If you have to haul clay, a bentonite delivery may be quite competitive.