As a person who visits tons of farmer's markets (some food vendors are my clients across the nation), I'm not exactly sure if vendors at farmer's markets who sell produce would specifically be your niche.

Generally speaking a farmer hits X # of markets per week to sell their produce. Selling small scale to individual consumers wouldn't work well for their time.

However I have heard of some restaurants in LA where they accept extra produce from neighborhoods for their restaurant (not sure if they buy it necessarily).