There was also a service called jammit marketed to musicians where you could buy multitracks from their partnered artists. There are also artists sharing them in deluxe editions of albums, for example I think Dream Theater did it for Black Clouds and Silver Linings?

There's also the possibility of just knowing someone in the industry that could have access to them, maybe? I know Rick Beato who does song analyses on YouTube is a producer, so he could get some this way.

There was a great channel on YouTube dedicated to sharing these isolated tracks called digital split, unfortunately it got banned from YouTube twice, so I don't think it's coming back...