Planting fruit trees in the fall is fine once the weather begins to cool and the plants you buy are at least starting to lose their leaves and going dormant.

As you said, it is more difficult to find fruit trees in the fall to plant because most nurseries wait until the trees have lost their leaves before they dig them up bare root. It is important to wait until the trees go dormant so they can store the maximum amount of carbohydrates in the root for winter. Most nurseries wait until December to dig and then start selling bare root in January.

If you want to plant earlier than that you often have to buy last year's bare root trees that did not sell and were potted up by the retail nursery. These trees are usually pot-bound and have larger girth in the trunk due to their age. They are also more expensive.

You will have to call around to wholesale nurseries to see which is digging bare root trees the earliest.