The hunt for ROMs can feel like a treasure hunt sometimes! I totally get the nostalgia for 'Sonic Underground'—it's such a weird, charming piece of Sonic history with that funky animated series tie-in. But here's the thing: downloading ROMs for games that are still under copyright is legally murky territory. If you're looking for a legit way to play, I'd check if it's available on official platforms like Steam or console digital stores. Sometimes older titles get re-released in collections.
If you're dead set on the ROM route, though, I'd at least recommend researching preservation-focused communities that prioritize games no longer commercially available. Sites like Internet Archive occasionally host abandonware, but even then, it's a gray area. Personally, I think the best approach is emulating games you already own—that way, you keep the spirit of the hobby ethical. Plus, hunting down physical copies can be part of the fun!