I think your best bet would be to get seeds from a plant growing on the Riviera (if such exists). The collections of succulents/cacti/lilaceae/agaves on some of those estates are quite extensive. If Hesperocallis Undulata cannot be grown in a rock garden there, then is a certainty that it cannot be grown in the Netherlands (without enormous effort).

Again, it's a long shot, but if you have friends who go to Marrakesh, Pantelleria, Ibiza, or Taormina, you might ask them to keep an eye out. Apparenty, though, the plant has very specific dormancy needs, and may not be able to adapt outside its historic range.

Another thought is to search the collections nearer you, and just down the coast....Biarritz, Jersey, Penzance and the Scilly Islands.

But thank you for alerting us to the existence of this plant. I'll look for it, when we're in the desert.

Oooooh! One final thought: when I image-searched Hesperocallis, the photos immediately reminded me of Camassia. Further research reveals that Camassia, like Hesperocallis, has recently been reclassified to the Agavaceae family. The two look like rather close cousins, to me. We had a pale blue Camassia growing wild under our Walnut trees, when I was a girl in Mississippi. I loved the plant: particularly the Winter foliage. And there is surely a variety of Camassia that will do well for you in the Netherlands.

Thank you very much for your answer and advices.
I do not know if this Hesperocallis is also growing in the Area;s you mention.
It is native in South west USA, California, Arizona and Nevada.
Of course I'll try in the area's you mention.
In September I am going on vacation in Marrakech, I open my eyes and ask there .
Yes, Camassia I knows, I have a few bleu varieties. Very nice plant.
It is a challenge to let grow this bulb in the Netherlands, in a greenhouse??

Thank you again very much and I hope , when you find this Hesperocallis, give me a sign

Kind regards

Ton