Cambridge International and Pearson Edexcel provide useful online resources for candidates, including past papers.
Cambridge Assessment International Education
You can search for support materials, including past exam papers, for IGCSEs, O Levels and A/AS Levels by going to the CAIE online resource centre.
Pearson Edexcel
You can search for past exam papers for International GCSEs and A/AS Levels using the Pearson Edexcel past papers search tool.