Not sure about Frontier, but I'm often asked to write recommendations for those applying to NP programs. I suggest you shadow an NP for a day or week to see what the job is like. At that point, have the NP write a recommendation for you. Of course, it won't highlight your clinical skills, nor is it someone you've had a long term professional relationship with, but it will show your interest in the job and your willingness to learn more about the position. Your other references should be from colleagues, supervisors, or educators.