The official ASUS eShops are almost the only retailers in Europe. That's why I was hoping to get an answer here. It's really difficult to get what we like even months after release. The only Twilight Silver model is the 6/128, there was never a 8/256 since May. 😕 I guess they are not even produced at all.
I'm based in the US, so I don't have access to much of that information. I don't know your specific region, as Europe has quite a few, but on your regional eShop page there should be a contact information section. The main directory is here https://www.asus.com/store.html. Give them a call or an email and see if someone in your region has access to that info.