Of course you could buy a nice plant at a nursery or garden center in the spring. The advantage is that you can see how the plant flowers. But remember that such a plant can be expensive. It is absolutely preferable to plant such a plant in its definite location as soon as possible. And that may pose a problem in spring, especially with frost.
Once the plant grows well, after a couple of years, a large plant can be torn. This is done in September. Not later, because then the bud formation is disturbed. Take up the entire plant, rinse as much soil from the roots as possible, and divide it into pieces that have at least three eyes each. Pot the torn parts or plant them directly at the place of destination. Give plenty of water immediately.
It is somewhat more time-consuming, but it is very well possible to grow plants from seed. The plants generally produce a lot of seed, which is ripe in late May or June. The seed will germinate around mid-December. In a very harsh winter the seedlings will probably all freeze to death, but in a somewhat milder winter they will survive and grow, during summer, into plants with three little leaves. These little plants can be lifted and planted in a different spot. After about three years they will flower.