Buying Fruit in Jam Quantities

Source: USDA. 

A question for you long-time Northwesterners: As my sister and I ogle the plum tree in back of her house and wait for another shot at making jam, we're talking about making a few batches of blackberry, raspberry, or strawberry jam.

It doesn't make sense, economically, to buy jam quantities of fruit at the farmers market, where a half-flat of blackberries can cost $28. Yes, we will could raid the public parks. But does anyone have a recommendation for a pesticide-free U-pick berry farm around here where we could get lots of fruit on the cheap? The thought of keeping my two-year-old nephew out of the brambles is daunting, but I don't mind trading my labor for a price discount.