We strongly advise to adopt a cat from a shelter rather than buying one. In some countries healthy animals can be still be legally killed after some days if they are not adopted, so your decision to adopt can literally save a life.
Below you find the links to national websites that list public and private shelters you can contact to adopt a cat.
Belgium
List of shelters
https://jadopte.be/refuges (French)
https://www.adopteereendier.be/ (Dutch)
France
List of "official" public shelters: http://www.30millionsdamis.fr/jagis/jadopte-un-animal/
Germany
List of public and private shelters (sorted by species) http://www.tierheim-deutschland.de/
Italy
List of public and private shelters http://www.prontofido.net/index.asp?action=05
Romania
List of public and private shelters: List of Romanian Shelters