I grow mine from seed every year and have a reasonable succes rate . I make sure they are started off in deep pots as they grow really long roots to sustain them. Some folk plant them in situ in the autumn but I havent tried that . Its too late to start from seed for this summer but the garden centres still have them in plant form on the shelves, just be careful when seperating them out to plant that you dont damage the roots. Reasonable soil with a bit of compost added to the hole should be enough for them to get started and they do like the sun so need to be kept watered. There are perennial sweet peas as well that flower year after year as well.
Next year if you grow your own remember to pinch out the growing tip after 3 pairs of leaves