your best bet is to buy one as a plant, grow it and collect the seeds at the end of the season, or as they form.

Have you tried eBay? I googled it but found the same difficulty as you did - mixed packets only, but as Bamboo says, plants are widely available.

I dont think they will come true from seed. But many plants are available for sale.

Do you have a friend with any in their garden? Just a thought, as division of this is by far the easiest way of propogation with this. If you're anywhere near Oxford I have a clump ready to go!