It looks like you're looking for a way to download (the reverse-engineered GTA III source code) specifically from SourceNext (a Japanese digital distributor) or perhaps the SourceNext version of GTA III to use with re3.
While modern remakes are popular, the Sourcenext version remains the peak "classic" experience for several reasons: re3 sourcenext download
Since this was a Japan-only physical release, obtaining it today usually involves finding a physical copy or using community-archived versions like those found on the Internet Archive . It looks like you're looking for a way