As others have said tagatose seems to be rare at the moment. Right now the cheap stuff works fine if you don’t combine it with carbohydrates.
The study was done where it was approved for use in food, in Brazil.
It is approved for use in foods in Brazil, alongside other countries like the European Union, Australia, New Zealand, South Africa, and Korea.
https://www.sciencedirect.com/science/article/abs/pii/B9780124046993000743