MonsterZero
I want to buy like 1 gallon cheaply.
I don't want to buy from Tamiya because they sell small bottles expensively...
German Armour
For tamiya use isopropyl alcohol.
modelmaker66
I use Tamiya acrylics, and through trial and error, I came to the conclusion to bite the bullet and use their acrylic thinner, X-20A. I buy it in the larger plastic bottle.
I started out like you; I followed my Dutchy senses, which made me want to spend as little as possible. So at first, I tried using water. I found that this didn't thin the paint properly, and that when applying it by hand, which I did for painting figures, the paint would clump on the brush and on the surface. When it did manage to apply well enough, applying a second coat would often lift the first coat off.
Then I tried isopropyl, but with the same result. Tamiya's paints and thinners are made with an alcohol, but it's not isopropyl. By that time, I was also using it for airbrushing, and I found the same problem as with water-it didn't thin properly, leading to clumping.
So I broke down and bought the bottle of Tamiya's proprietary acrylic thinner. And all of a sudden, the paint thinned and went on beautifully, whether applied by hand or with the airbrush. And when applying by hand, I can put down coats as thin as if I had airbrushed them.
I can't speak for others' results, but that's how mine came out.
The one exception to using Tamiya's thinner is using lacquer thinner when airbrushing. That also thins the paint properly, and it has the added effect that the matte paints are even flatter than when thinned with the Tamiya thinner.
And a little bit of thinner goes a long way.
As for Model Master acrylics, those I can thin with water or with isopropyl, and they go on just fine.