The meal at the inn is free, by the way, and you can eat there daily. Once you've saved Sigurd, your food troubles should be pretty much over.
For food on the go, here are some sources (very minor spoilers):
*
Hunt: when you kill a varg, use the ice pick on it to carve out some meat
*
Fish: at Lake Gandvik, you can find a fishing net AND a fishing hole. As a sorceress, you will need *luck* to catch anything.
NOTE: The meat and the fish are raw, you'll need to cook them to be able to eat them. They don't spoil, so go ahead.
*
Buy supplies: Sigrun sells SPIM rations and apples
*
Mushroom hunting: in a certain area of the game later on (from chapter 2 onwards), you will find edible mushrooms (although a certain someone will tell you they are poisonous, they actually aren't).
*
Hoard freebies: you will be given free supplies by the Jarl at the beginning, and some additional food at the start of chapter 3.