Mine come in bare root in next couple of weeks (they're usually lifted after a frost as the leaves drop, but in any case by the second week of November). That's a good time to buy them, and if you can, from a reputable nursery where the stock is freshly lifted and undamaged, so that you can be sure of the best start when they're planted. Have your site prepared, well cultivated, and get them in the ground as soon as possible to limit the drying out time for the roots and so they can make some new root to get established while the soil is still relatively warm. I aim to sell most of my bare-root stock before Christmas, and the rest gets potted up.

Hortum cret's information is spot on, but I would like to add this.

1. If it's possible try and get a named cultivar from a reputable nursery.

2. You might want to consider a grafted tree. This is created by grafting a part of one tree onto another tree's rootstock. The rootstock, which would form the lower part of the tree, would be chosen for growth vigour, disease tolerance or to create a dwarf tree, etc. The grafted, or upper bit, would be selected from a tree that produces best fruit production, etc.