Is it too late to think it's possible to get a summer Step 2 CS date? Two weeks ago there were July openings... ? If this wasn't medical school, I would think it was a CSI episode....
The thing with booking CS dates during the summer is that you have to be checking constantly for openings. They will happen, you just have to be flexible. Typically people will cancel 1-2 weeks prior to their exams, so thats a good interval to look for. This is only based on my experience.
I would book a tentative date and then keep checking. I think you can sign up to get notifications whenever a spot becomes available. And like @palakas said, people will cancel 1-2 weeks before their scheduled date.