Hi,
Exclusiveness - tendency to associate with only a select group (and often used in the classical literature).
Exclusivity -
contract term in which one party grants another party sole rights with regard to a particular business function.
Within the limitation of my knowledge exclusivity is the legal term. Interestingly exclusiveness and exclusivity are not synonyms!
Maybe you can try
http://www.legal-definitions.com/, where you can find legal definations.