Every community is a little different, and every grocery store and farmers market is too. Sometimes you find "hidden treasures" in unexpected places like tiny markets, roadside stands and Mom & Pop grocery stores, other times you'll find the most variety in the largest grocery store, or the grocery store nearest the most affluent side of town (or closest town with an affluent community).
When it comes to finding the most produce diversity, I'd suggest starting your hunting at the farmers markets and farms.
We're coming up on fall harvest festival season, which not only are great sources of produce, but also local entertainment. Local newspapers and community flyers and the Small Business Association and Chamber of Commerce (and of course Google) can help you find these local events.
Many small farms also have festivals and other touristy events scheduled in September through November for tours, hayrides, pick your own, corn mazes, and even petting zoos and such.
Try searching for your nearest farms and apple orchards with visiting hours or onsight stores. Visiting apple orchards (at least here in the Midwest) is a common fall entertainment, and most sell not only apples but squashes, corn, and other locally grown produce as well as baked and canned goods. I'm a bit of a condiment freak, so I love gowing to these places just for the pickles, jams, relishes, salsas and mustards.
Grocery stores (even health food grocery stores) often tend to carry the most popular varieties and/or those that travel/keep on store shelves the best. The hardiest veggies for transport aren't always the tastiest.
For the best variety, farmer's markets can't be beat. In very large cities, there may be a dozen or more farmer's markets to choose from (and each will have it's own "personality.")
In my area for example, one farmer's market has a bit more of an upscale reputation (which means ironically enough that the the prices are higher and the selection and quality are lower).
The selection, freshness, and prices are much better at the farmer's markets that have a high percentage of lower income shoppers as well as more cultural diversity of both shoppers and vendors.
Don't be afraid to tell the vendors what you're looking for, even if they don't seem to have it. They may tell you where you can get it, or they may even grow it, but don't bring it to that particular market because it doesn't sell very well there. I've actually made "deals" with vendors for them to bring produce I'm looking for to the next market day, or for me to visit their farm where they sell more than they bring to the farmer's markets.