I paint with Golden matte fluid acrylics, thinned with Liquitex ultra matte medium and water. I used to use Golden's matte medium, but it just wasn't matte enough.
I fell in love with flat latex house paint when I was in grad school, but decided to look for a higher-quality paint that would give me the same effect.
I have been obsessively seeking the "matte-est flatness" possible in acrylic paint, and have settled on what I think is the best combination to get the look I want.
So far, the largest size I can find for the Liquitex is 16 oz, which is available online at Daniel Smith. I can get the 8-oz. size through Dick Blick, along with the 4-oz bottles of Golden Matte Fluid Acrylic. For larger sizes of the acrylic, I order online through The Italian Art Store.