I wish I knew.
Going on the Internet and on YouTube since yesterday, everyone seems to have the right solution.
Windshield washer, Windex, Future, homemade thinner, etc.
What I know is that I will have to experiment since I have lots of different acrylic paints and the last thing I want is to clog my airbrush and put it in its box for the next 30 years.
I still have a jar of my precious Tamiya paint thinner which worked fine the last time, but someone is selling it on Amazon for 57$.
This is not an option. Since I bought some items for my airbrush yesterday, I have time to ponder over this before finding the right solution.