marycontraryquite practice papers are invaluable but you can get them for free from the exam boards and the teachers give them too (sometimes charging photocopy costs onto parents).
DS didn't really have time to do any extra papers, tbh. Except for science. Maybe if he had started earlier...?
Before you buy things, check what the school arranges. They often get bulk buying discounts, sign your DS up for subscription services etc.
For science, DS got the CGP revision guides, plus the question and answer books. He used them a lot, until I signed him up to Tassomai (April)