Finding a solid sample of the first chapter for a business proposal book really depends on what you're looking for. Are you after a traditional, formal business writing guide, or something more modern like a startup pitch handbook? My usual method is to hit up the big ebook retailers directly. Amazon's 'Look Inside' feature is the most reliable for this, hands down. You don't even need to download anything; you can just read the preview right in your browser. I used it to check out the opening of 'The Art of the Pitch' before committing, and it gave a perfect sense of the author's style.
Beyond that, a lot of authors and business coaches offer free chapter samples as lead magnets on their own websites. If there's a specific expert you follow, it's worth signing up for their newsletter—they often send the first chapter as a thank you. I've collected a small library of first chapters that way. The downside is you have to trade your email, but for a niche topic, it's often the only way to get a sample beyond a generic retailer snippet. Just make sure you're on the actual author's site and not some third-party content scraper.
Honestly, I've mostly given up on dedicated 'sample' websites for business content. They're either outdated, push you toward shady PDF downloads, or have such a limited selection it's not worth the hunt. Sticking with official storefront previews or the author's own platform is cleaner and more likely to get you a current, legally shared excerpt. The search can feel frustrating, but that first chapter is crucial to see if the advice resonates before you spend on the whole guide.