Do a Gogle search Sellina and you come up with lot of places to buy both seeds and plants.

Most people grow this from seed in early spring,starting them off indoors - this plant, although its a perennial, woody vine, is listed as H2 for hardiness, which means it absolutely does not ike temperatures lower than about 5 degC, so in the UK, this is usually grown as an annual. You may or may not find seeds available now, but Wilko usually has them in spring.

That said, Chiltern Seeds online has them for £1.95 a pack, there's probably other online suppliers too.

Botanical Interests sells the seed, but I don't know if they sell in the UK.

Hi Tug. no, they don't.

Oh, well. I've probably been told before. maybe this time, I'll remember! :)