Try the Parts People https://www.parts-people.com They ONLY sell Dell parts and are located in Austin Texas.

They know Dell and are reasonable priced.

Just about the only place you'll find these is on EBay (or similar).

https://www.ebay.com/p/Dell-Alienware-Area-51-R2-Case-Chassis-Build-Your-Own-Gaming-Computer/2264966367?iid=183116829990

Thanks for the replies. Is there no place in Europe that would sell these cases? 

Most manufacturers don't permit established third-party businesses to sell complete cases -- you will do better searching EBay (you can limit it to Europe) -- or look for someone selling an older model, which may turn out to be no more expensive.