Had to look that one up as it was unknown to me. Seems to be a lactiflora x macrophylla (probably tomentosa) hybrid by Saunders. I can’t really think of an easy source for that one in Europe. There are some specialty peony nurseries in Europe that list several Saunders hybrids. Ruud Warmerdam at http://www.paeoniapassion.com or Daniel Giessler and Anita Litschke at http://www.giessler-paeonien.de or Walter Good at http://www.paeonia.ch . All of them have extensive lists of rarer peonies and sell good quality plants (I can tell by experience). They all have (far) more varieties than those that are listed on their websites (Rose Crystal isn’t listed on their respective websites currently). Usually that is because they are building up stock of those varieties they only have very few plants of. So it might be a good idea to simply write them an e-mail to ask for that variety, it’s a good time to do so as the new season 2021 is barely beginning and availability will be at its’ highest. Alternatively you might want to contact the American Peony Society to see if they know a source. I see that Rose Crystal does have an image in their registered cultivars database. Most of those images are from Henry Chotkowski @henry-chotkowski who grows many varieties, thus probably at least he has it or knows where to obtain it. You can easily find him on facebook, although he’s also registered here, but not sure if he visits here often (I see he’s registered twice). There are also quite a few nurseries in the US that list Saunders peonies, probably more than in Europe, which is to be expected of course as Saunders lived and worked in the US. Alternatively you might try Chalice, which is of the same cross, but more easily available. Or Garden Peace, somewhat further hybridized, but a fantastic plant, large glossy healthy foliage, sturdy tall stems and very attractive buds that develop into very large perfectly formed flowers. Within the singles one of my favorites. Good luck!