I’ve finished the beginner 30-day courses for barratones and I’m starting to work on the next level. I’d like to keep practicing the warm-ups and techniques, but without relying on another singer’s voice as a crutch. Is there anywhere I can find MP3s of the exercises with just the piano accompaniment?

That's a great idea, Alex! Although I don't think keyboard-only tracks are extracted from the lessons, you can actually make your own here by uploading the exercise mp3 and using the "splitter" tool: https://vocalremover.org/. 

Great, thank you!!