You can find used physical copies for PC, GameCube, Xbox, and PS2 on marketplaces like eBay or Amazon. Note that the GameCube version is widely considered the most stable, running at 60 FPS with fewer glitches.
Part of the SEGA Forever collection, available for free on mobile devices. Sonic Heroes Free Download
Since the PC version is no longer sold by SEGA, many players turn to community-vetted sites like MyAbandonware or OldGames.sk. You can find used physical copies for PC,
