George's sample chapters have been scattered like breadcrumbs over the years, not all in one place. I'd start with his official Not a Blog where a few were originally posted, but a dedicated fan site like Westeros.org keeps a comprehensive, updated archive with every released fragment, including the readings he's done at conventions that never got a formal post. Some are even transcribed from those live events. It's a bit of a patchwork, but that's part of the fun for the super-fans. You can trace the evolution of certain chapters across different drafts he's shared. Just be aware, some of those older sample chapters might be from versions of the story he's since changed, so take them as a glimpse into the process, not necessarily the final product. I reread the 'Mercy' chapter from Japan last week and it still holds up as wonderfully unsettling.
Avoid random blogs that might have messed-up formatting or missing paragraphs. The big fan forums usually have stickied threads with clean copies. And of course, none of this is the actual book, which feels more like a collective act of hope at this point.