Hi - while you can find free resources, in my experience you should at least buy the books.
For both of the ISC2 certifications I passed, I only used the study guide and practice questions offered. While you can take the ISC2 Self Study course that costs a lot more, you can get by with just the books if you're really focused and have a good plan.
You can get these books on Amazon at substantially less than the list price. All in with sales taxes included, I think I paid maybe US$145 in total for all three study guides I used - Dummies, Official Guide, and Official Tests. (The test itself was way more expensive at US$600.)