There are a ton of apps out there that show you where the nearby restaurants, hotels, or other tourist locations are in any given city. My idea is a bit of a take on this where the focus is on where locally grown produce, meat, or drinks, etc. can be purchased. This would give local farmers or roadside stands an opportunity to "advertise" to people who may not be familiar with the area. I can see this being expanded to include local vintners, brewers, or similar local food/drink producers. This could also be a great opportunity to community gardens who might have roadside stands.
My thought is that there would be a search engine where you could search for a specific food product to see where it might be available or something broader, like "produce".