We've been on a few tours in Thailand. Of the private ones we had a good experience from here http://yourthaiguide.com. They are very responsive and accomodating. We tried e-mailing with Tong last year. She must be very popular because she was not responsive at all. We also tried Chob Panma's company. Our guide, Ms. Get was just okay. They were very responsive and organized though and have a lot of guides working for them.