Buy a used physical copy from eBay. It costs less than a movie ticket and a coffee. Hold the maroon cover in your hands.
Note to readers: This article does not host or link to copyrighted PDF files, nor does it condone illegal distribution. It exists purely as a historical and practical guide for hobbyists. 6th Edition Space Marine Codex Pdf
Look for the compiled data sheets (text-only) online. You need the rules, not the art. Buy a used physical copy from eBay