Most shrubs sold in nurseries and garden centers are "containerized" --- having been grown in a black plastic pot. Shrubs have traditionally been sold by their pot size, which is in "gallons" --- 1 gallon, 2 gallon, 3 gallon and 5 gallon.
Some containers use a number (#2, #3, etc) designation instead, but the concept is the same with the number indicating the volume of the container. Shrubs grown in pots can be planted nearly year round, since there is much less disturbance of their roots than if they were transplanted from the ground. Field grown plant material is usually limited to being dug during the dormant season, which is spring or fall.
Only buy shrubs that look healthy and vigorous. Sick plants, even at a discount, are no bargain.
When transporting plants, try to protect them from getting "wind whipped". Covering them with a fabric tarp works very well. Avoid dark plastic wraps that can heat-up and scorch leaves.
While holding potted plants for future planting, pay close attention to watering. Black plastic pots heat-up in the sun causing the soil to dry out quickly. Most nurseries water pots daily in the summer.
"Circling roots" are common in potted plants. Be sure to loosen them with your fingers, or slice the root ball on two sides and across the bottom with a knife. Circling roots can become girdling roots, causing future problems.
Most shrubs should be planted at least 3 feet apart, and 2 or 3 feet away from walks, steps and buildings. Plant spacing that looks good right away may result in crowded beds after 5 years. Allow room for future growth.
Pay close attention to watering your shrubs during their first growing season, as well as during droughts in future years. Summer watering contributes to next year's flowers and vigor.
Keep an eye on your shrubs for any signs of insects or disease. Clues to watch for include "off color" foliage or a "spotted" look. Always READ THE LABEL of pesticides BEFORE you use them.