I sent a letter to all the DOs within a 30 mile radius of me (there were only 12 or so). I knew I'd have to travel a bit to shadow them. In the letter I explained who I was and what I was looking for. Two got back to me, and I ended up shadowing one of them over the course of a few months. I explained upfront that I would be asking for a letter from him. He was very excited about it, and he was very open to helping me with anything. I had called and emailed a bit before, and I had no luck. I figured with a tangible letter, they could respond when they wanted to, and it wouldn't get tossed out in the "unimportant message" pile. They're busy people (everyone in the office including the receptionists/MAs, etc) so just be gracious.