I'm currently applying to Baylor for graduate school (in nursing).
I'm in a similar situation: working agency so I don't have any dependable ties to supervisors; the agency recruiter isn't a nurse and really doesn't see me on the job.
The professors I liked and trusted well enough to ask are now retired and apparently have changed their email addresses....
My best shot, other than peers who are nurses, was my veterinarian, since he and I spend a lot of time taking care of my animals, and he teaches along the way.
I think the admissions assistant at Baylor chuckled when I asked about using my vet as a reference, but I cannot be absolutely sure.
They suggested that I get references from "nursing peers and colleagues." This would, according to her, also include nurses that have worked for me when I was myself a supervisor.
So I have a classmate from Excelsior that I have gotten to know but with whom I have never worked, a nurse practitioner who I consider to be a mentor but with whom I have never worked, and the nurse I supervised for six months or so.
You might do what I did: call the school, explain your situation and get their guidance. Also make a note of who you talked to, what they said, and the date and time (I usually send myself an email with these particulars). Then later, if your memory or theirs goes "on the fritz" you have your record.
Good luck, and welcome to nursing!