Buying specimens in three-inch pots is the most usual way of obtaining new plants. You can also propagate your own or receive them as gifts, but you should never dig them up from their natural habitat. Most garden centres offer the widest range in spring. Call to see what is available and what is in bloom at different times of the year. Look for a good example of the variety you have chosen. Keep it in a shady spot if you cannot plant immediately and water daily.
Specialist nurseries carry a much larger range of rock garden plants than general-purpose garden centres and skilled advice is usually available. This is the place to go to if you are looking for rarities, but you cannot necessarily expect to find an alpine nursery within easy driving distance. Many offer catalogues and mail order service, deliveries are generally made in spring and autumn. This is not the cheapest method; you have to pay for postage and packing. Remember to unpack immediately and wait a few days before planting.