Never pay money to 'Trekking' agencies based in Chiang Mai, or anywhere in Northern Thailand, to take you to have a hill-tribe experience. You are being grossly overcharged by unscrupulous Thai middlemen, who are exploiting these unique ethnic minorities for their own means (they get very little if anything from these trekking 'guides'). Its really not that hard to find them yourself, with a good map, bike and some courage & sensitivity.
Do your research beforehand. Trust me you'll have a much better reception when (if) you find them, plus your overall experience will be much more genuine. Take gifts of course (food and pens are welcome), if you are invited to stay overnight offer cash only to the head of the village on leaving and its fair to give the same amount that you would have paid to a hotel/gh/restaurant for the same.
Forgive me for being blunt but I have spent a lot of time first hand seeing the effects of this type of tourism and all I can say is that if all this seems too much effort then leave them well alone.