Very common problem, particularly with Schirmer scores. Even if you had the rehearsal numbers, it may not be what the maestro is conducting from anyway.
I will guarantee that the pianist is not playing from a full orchestral score (if I have to eat my words, that person is a musical genius, or Cosi is easier to play than I know). At the very least, you may be able to get a copy of what that person is playing from if it is a different publication. I will also say that he/she and the maestro have already or will at the beginning of the rehearsal process go thru the "can I borrow your score to get the rehearsal numbers over the lunch break?". You can then ask to borrow the pianist's score that night, to put them in yours. Alternatively, if this is a company with adequate music staff, you can ask this same question during prep. They should know what publisher the conductor and orchestra parts are from, which then determines how the maestro will be looking at rehearsal numbers.
For those who don't know, sometimes the measures are numbered consecutively (242, 243, 244, etc.), but more common are large numbers, often in a square or circle, above the staff. These are called rehearsal numbers and are sometime rather arbitrarily assigned by a publisher, although hopefully they're at the beginning of a change in music, chorus, or other really good place to pick up from in rehearsal. When doing opera, it's helpful to go thru the score during prep and highlight the rehearsal numbers, so you can find them quickly.
It's important to have what Maestro has, so when you hear him tell the orchestra "rehearsal 42", you're already on your way to being ready for him. (or her, not to be sexist here...)
I hope that helps Scooter! Even if you don't have them before rehearsals start, you'll find a way to get them before tech, I'm sure.