I'm a freelance writer and yogi living in Tepoztlan, Mexico where I'm running www.casadelcorazon.mx with my girlfriend. You can keep up with my other writing on www.bollocks2thewellingtons.com/
On Moon Muang Soi 6 is a little market selling vegetables, fruits, snacks, some clothes and make-up and other bits and bobs. It's often visited as part of cooking courses as the market tour part of the day but living in this area it's invaluable for a snack of fruit or to buy things to make your muesli in the morning.
There are a couple of stalls selling many types of Prawn Crackers, nuts and crisps for those long mini-bus rides or you can get yourself some Chiang Mai Sausage to keep you going on your trip around the old city.
The prices here might be a little higher than some of the local markets due to the influx of Farang but you can bargain and it's still pretty reasonable.
If you're unsure as to what some of the local vegetables are you can always linger around a cooking class group and listen in to the guide who will explain some of the more exotic ingredients...