The artistic director, generally also the conductor, has the most time involved in the production. Thats my starting point, if I can find that persons contact info. They always appreciate hearing what is heard in the hall. They get locked into the moment, go musically mechanical, and don't hear the concert the way that the audience does. So they appreciate a recording that lets them hear what they've produced, and missed.
Most orchestras will have a website. Contact the info@xyz email address; get ready for no answer. If no answer, try again. Try to establish contact, and/or ask to be redirected to the conductor. I've had more than one occasion where permission is not granted until the day of, or days before the performance. Start early.
If permission is granted, *be there a couple of hours before the concert, to meet, greet, and talk to the hall staffers, stage manager, the conductor, the concert master, and anyone that you can.
Be kind to the stage manager. He has as much say in your success as anyone.
Also get ready for plenty of rejections.
* I've yet to meet an orchestra that doesn't have a final rehearsal in/for the last couple of hours before a show. This is a prime time to be already set up and ready, and get a soundcheck. Your dynamic swings will be massive, far beyond PA recording. So, having a solid soundcheck behind you will be a huge benefit. In that time, you can speak to the conductor, and have him give you a large crescendo to set levels to.
Cable dressing,... being a tight profile makes you look like a professional. Tape everything down, velcro cables to the stand. Make yourself look like a pro.