For my second straight trip to Chicago, I had plans set to meet a well reviewed provider (different ones on each trip) and both times, even after confirmation early in the day, the appointments were cancelled for understandable reasons.

Now, I'm left scrambling and having NO luck finding someone available...what's the best move to make for someone on short notice?

I would say have a backup girl and get pre-screened by her and just be honest with her and let her know u will contact her if your other plans don't happen.  That way you have last minute options.