Personally, I think the pre-packed trail mixes are a waste of money. They're wildly expensive for what you get. You can go to any decent food co-op or even a good, big grocery store and get all the stuff you need in the bulk containersnuts, seeds, dried fruit, small candies, you name it. Forget the Web, try this recipe from Chez Gear Guy, for instance:
In a bowl combine one quarter cup of any eight of the following items:
Unsalted nuts, peanuts, cashews, or almonds
Mini chocolate chips or coated candies
Plain or multi-grain Cheerios
Whole-wheat Chex cereal
Store in an airtight container, pack your bag, and you're ready to hit the trail!