Reborn is one of those classic shounen anime that flew under the radar for a lot of folks, but it's got such a dedicated fanbase. I binged it years ago when it was still airing, and tracking it down now can be a bit tricky since licensing shifts over time. Last I checked, Crunchyroll had a decent chunk of the episodes, but not the full run—some regions might have more complete libraries than others. Hulu used to carry it too, though their anime catalog changes frequently.
If you're open to less official routes, some smaller streaming sites might have it, but quality and subtitles can be hit or miss. Honestly, I'd recommend hunting down the DVD releases if you're a physical-media person; the dub's fun, and the filler arcs are easier to skip that way. The manga's also worth a read if the anime's hard to find—the art really shines in the later volumes.