I totally get the hunt for 'Bungou Stray Dogs' chapters. First thing to check is always the official sources—Viz Media usually has the latest few chapters of their Shonen Jump series free on their site or the Shonen Jump app. You might find the most recent 3-5 chapters there legally. The manga's serialization status is a factor; because it's an ongoing series, free official releases are often a 'latest chapters only' model to hook you into buying the volumes or a subscription.
Beyond that, the landscape is a mess of fan scanlation sites that pop up and vanish. I've used sites like MangaDex in the past for older chapters, but their availability depends entirely on which scanlation group is currently working on it and if their uploads haven't been taken down. Quality and translation consistency can be all over the place—some groups are fantastic, others are nearly unreadable. It's a real gamble.
Honestly, if you're just starting, I'd say the official free chapters are the best entry point for readability. If you're trying to catch up from the beginning without paying, you're looking at a patchwork of different sites, and you'll probably end up with a dozen tabs open. I remember trying to find a specific volume's worth of chapters and bouncing between three different aggregator sites because each one was missing random pages.