You intend to use the Kubler Ross model of Grief in one of your publications Do you need to get copyright permission from the EKR Foundation?
Not if all you're using is the idea and not the actual EKR text. However, you must credit EKR properly to avoid plagiarism. You may also use excerpts of an EKR publication as examples in scientific inquiry, as this would be a permitted "fair use", to the extent you use only as much as needed to make your point.
However, the EKR Foundation may be more than willing to permit you broader use, under a one-time license, provided you ask nicely and agree to indicate "used by courtesy of...". It rarely hurts to ask.