While corn starch or flour may well thicken it up, I'd be hesitant to add it to paint because it's likely to affect the longevity of the painting. Also, when you mix it with water it makes a hideous, slimy paste that I can't see would be pleasant to work with. I'd personally opt to spend a little money on a texture gel or modeling paste that's made with the same resins etc. that acrylic paints are, and then you wouldn't have to worry about it.
Check on the label to see whether the gel or paste will dry clear or opaque, matt or gloss, and whether it will influence the color of the paint if you mix it in. Some pastes look white, but dry clear; others have fillers in them that influence the intensity of the paint color.
Texture gels or pastes are water based, so it's easy to clean up your brushes or painting knives after using it. You can either mix the texture gel with your paint, or use it to build up texture first and then paint over it. Some you can even carve back into.
Various acrylic-paint manufactures produce such pastes, with a price tag related to the brand. Something such as Winsor and Newton's modeling paste in their cheaper Galeria acrylic range for starters..