You can find heirloom veggies and less industrialized produce in American markets — but part of the reason they're not quite as popular is that they don't ship as well or have as long a season. Fuerte avocados, for instance, have a season of about six weeks. They're delicious but towards the end of the season they're not that great.
Of course, living in the Bay Area I'm spoiled by the selection. It really is better than a lot of other areas. In contrast, Portland, in particular, stood out as having consistently mediocre produce available (and New York was surprisingly good even in the middle of winter).