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: http://www.chien-admis.be/pages/mes-pages/refuges-chenils-et-societe-protectrice-des-animaux-spa-srpa.html

France

List of "official" public shelters: http://www.30millionsdamis.fr/jagis/jadopte-un-animal/

List of rescue organisations that offer cats for adoption: http://www.secondechance.org/refuge/list?espece%5B%5D=1&Region=&Departement=&commit=Rechercher+%21

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