MasterShoestring is going into Year 11 in September and last year when he started the new English course I got all the set books second hand via the internet. I saved a substantial amount of money compared to buying new and he had the same texts as everyone else.
He is doing the new GCSEs graded 1-9 and since this year was the first year of the new exams it wasn't possible to get any of the revision books second hand. However, with the results about to be published lots of people are about to be wanting rid of their GCSE revision guides and now is the time to watch out for them at good prices on book selling sites or even for free on Freecycle.
The other way we have saved on set texts is to order through the school, which gets a discount on a bulk order, this is especially good for the CGP guides and using Google Kerboodle which gives you access to textbooks and revision material for free if the school or yourself takes out a subscription. This is particularly good for Science books.
Good luck to everyone waiting on their results at the end of August and to all those going into Year 11!