I was in a similar situation- went into nursing after a number of years in the tech sector.
When I was applying for my nursing assistant job just before my 2nd semester of nursing school
, I asked an instructor if they would be a reference, and I listed two of my previous employers with the 'central' telephone information number for them- my old managers had long since flown the coop (same as in your industry), so there was no way i could find them. I also told this hiring manager that it would be difficult to find my actual former supervisors, but the HR dept. could verify my employment.
So, I offered personal references- including a former coworker from one of my tech positions with whom i'd worked closely. I offered up five or six personal references, since the business refs were such a problem. If you're still in touch with any old coworkers, I'd try that. When I did HR for a LTC place, I did two things- verified employment (which went through the HR department at the prior employer), and actual reference checks- sometimes those were former supervisors, sometimes coworkers, sometimes long-time friends.
Another idea would be to list people you've been involved in volunteer organizations with- especially if you were involved specifically in some sort of project with them. Or perhaps a church contact, if you've been active in some project there.
The bottom line is that, IMO, most employers will be okay with other types of references instead of the 'past three supervisors' they usually request. Frankly, with so many people coming into nursing from high-turnover careers- and the geographic mobility most people have these days- i think only a minority of people could actually find contacts for previous supervisors in another industry.
One thing I would advise, though, is if you do any of the things I mentioned above, make sure the person actually *checking* your references understands, not just the hiring manager.