When trees are still in leaf in late summer into early fall, you can only find and plant trees in containers. Sometimes nurseries and big box stores have sales to unload such trees, but selection tends to be poor and the trees may have been stressed a great deal by restricted root growth and neglect, especially from inadequate watering.
Late in the fall some nurseries sell bare root trees which can be planted as soon as they become available- to some likely advantage in early establishment. Nurseries list - #72 by SpudDaddy
Nurseries that specialize in fruit trees are more likely to have a selection that includes the best varieties for your location (especially nurseries located closer to you) and are more likely to sell properly identified varieties.