I buy my clematis from Raymond Evison website. He has won numerous awards and the plants are superb quality and renowned for the flowers and long flowering periods. (pricey, but highly recommended) Taylors Clematis website is good as well (but again pricey)But with both the above you will get a well established 2 year old plant. You can buy 'baby' clematis cheaper from Thompson and Morgan but will probably have to wait 2 years before you get any decent display flowers. I bought 'Shimmer' clematis from Raymond Evison in May last year and it flowered from June to October and was tremendous. Raymond Evison clematis are designed for container/pots. My clematis are in pots and don't particularly get full sun. Hope info helps ! ps stop feeding your clematis when the flower buds are the size of peas (its prolongs the flowering period - just give them water)

Nearly all my clematis are bought from supermarkets, which once established have always flowered for me, plant at a 45 degree angle and make sure you plant 4" further down than you buy it to prevent clematis wilt, they also love a cold foot, I always have placed a peice of roof slate over mine the saying goes feet in the shade head in the sun. Spring or Autum would be the best time to plant

To find out what online suppliers there are i would google that subject and take your pic !
I just did it and several came up.

If you're going to grow them in pots DO make sure that the pots are at least 2' in diameter and AT LEAST that in depth, remember, they have long roots and hate being disturbed - so the pot you plant in will probably have to be its permanent home.

Your position sounds fine because they don't need full sun all day long and they prefer roots to be shaded by something anyway and then have their heads near the light/sun.