Almost all tour agencies in Northern Thailand advertise tours to hill tribe villages, unfortunately most of these are to villages which have been modified for the tourist, so the experience is not in its natural setting. These villages typically charge an entrance fee and stands are set up inside where villagers sell souvenirs and handicrafts. Even the souvenirs and handicrafts are not handmade by themselves but sourced from town. Some also argue that the hill tribes are being exploited by the company that manages the entrance fees, and do not receive a fair pay.
With a little bit of research it is possible to find tour agencies that offer an authentic and ethical experience of the hill tribes.