You have a choice of bare root, balled in burlap, or in container. Most of what you buy will be in container. Some nurseries will still do balled in burlap, particularly for bigger material. Container sizes are regulated by ANSI. Select healthy material. Look at it very carefully before you buy it. You want a single leader even on your hardwoods. And you don’t have to be a plant pathologist to tell if a plant looks healthy. You should not see any wounds, and any sort of open cankers or sores or swelling (which is a gall). Avoid girdling roots. So you want to look over your plant very carefully and if you see problems, then don’t buy it.