If you are interested in particular clematis cultivars, it's really best to purchase nursery plants, rather than growing them from seed. Nursery plants are clones of that variety, and will come true (in other words, they will actually look like the variety you want). When you grow named cultivars from seed, there is no guarantee that they will resemble the cultivar. Self-seeded clematis in my yard have a huge variety of flower forms and colours... not (Edit!) many look like the original cultivars.