Doris, I always buy from an owner-run nursery if possible. It isn't because they necessarily have healthier plants, but because someone who wants and needs your repeat business is much more interested in your gardening success than a large retailer.
No, I don't think that the smaller plant stores have healthier plants. You can judge the health of a plant for sale by looking at the foliage, seeing if it needs water, if it is "leggy" or stunted. Next to fingers that weed, an experienced eye for plants is a gardener's best asset. The experience comes from, well, experience!