Man, this is a question I see pop up constantly, and honestly most people immediately jump to the big retailers, which is fair. Amazon Kindle's 'Look Inside' feature is decent if you want a controlled sample, and you can usually read the first chapter of most spicy titles there. But honestly? That's the curated, safe-for-public version they want you to see first. It doesn't always show you the actual heat level or the writer's style for the good parts. If you really want to know if the 'spice' is your type of spice, you need to go a step further.
For me, the real treasure is authors' own websites and their mailing lists. Almost every author I've ever subscribed to gives away a freebie sample novella or a couple of steamy chapters just for signing up. It's a longer sample than you'd get anywhere else, and it's usually more representative of their actual work, because they're giving it directly to their core audience. Sites like BookBub are also underrated for this—they aggregate free and discounted ebooks, and you can snag full short stories or the first books in a series for free, which is basically an extended sample.
Just watch out on social media, though. Those 'spicy excerpt' reels on TikTok or Instagram are edited for maximum impact in 30 seconds and can be super misleading about the book's overall pacing. I got burned a couple times thinking a book was non-stop action based on a clip, when it was actually a super slow burn with one explicit scene three-quarters of the way in.