If you don’t have a regular ophthalmologist you *might* get reasonable attention at an ER (yes, I know how awful that sounds) - especially if you can go to a teaching hospital. They’ll generally have an ophthalmologist on call, but it could be a long, expensive process. Alternately you might get results by pressing for an urgent referral by the doctor who is telling you to stay on prednisone.
I hope the whole thing turns out to be no big deal, but hope isn’t a strategy.