Between the post office (Radio City Station, methinks) and a parking lot on the south side of 52nd Street (8th & 9th) is Stiles' Farmer's Market. It's basically a plywood shack covered with a tent.
They have about the same selection as a good manhattan supermarket, with slightly better quality at same or slightly better prices. On Wednesdays in the summer you'd be much better off at the mini-Greenmarket at 57th & 9th, but today it hit the spot.
They also carry coffees (beans) and a few fresh bakery items.