You find below direct access to websites of non-governmental-organisations that plays a major role also in the fight against corruption.

• AIM (with a dossier on Corruption in the countries of South-Eastern Europe)
• Anti-Corruption Network for Transition Economies
• Association of Certified Fraud Examiners
• Center for the Study of Corruption and the Rule of Law (CSC)
• Coalition 2000 (Bulgaria)   
• Freedom House 
  • Homepage
  • Country Reports "Nations in Transit" (2001, 1999-2000, 1998)  
  • Press Freedom Surveys
• Global Policy Forum
• Independent Commission Against Corruption (ICAC), New South Wales (Australia)   
• International Association of Judges / Union international des magistrats
• International Association of Prosecutors (see Recommendations on Combating Corruption in Public Administration)
• International Bar Association
• International Chamber of Commerce
• International Commission of Jurists
• International Consortium of Investigative Journalists
• Internet Center for Corruption Research
• National Legal and Policy Center - promotion of ethics in government (USA)
• National Whistleblower Center (USA)
• Reporter sans Frontières
• The Southeast European Legal Development Initiative (SELDI)
• Transparency International
  • Homepage
  • National Chapters of Transparency International  
  • Annual global corruption report
  • Special projects on corruption and political financing / projets spécifiques sur la corruption et le financement politique
• UNICORN
  • Reports
  • Corruption Archive
  • Corruption Sources
  • Whistle blowing

Source:
Group of States against corruption / Groupe d'Etats contre la corruption
https://www.coe.int/greco/