I use Tamiya's acrylic thinner. Although more expensive than water or alcohol, because I use it only for thinning, a large bottle lasts quite a while. Mixing ratio is a bit problematic because different bottles of paint have different thicknesses. The old rule of thumb is mix until it is the consistency of milk. Until you get the hang of it, add thinner and test until it sprays nice. For me, I find that too much thinner is less of a problem than too little thinner.
Another odd quirk to be aware of is that the thinner has a tendency to evaporate out during a painting session. Probably not much of an issue when laying down an overall base coat, but if you painting camo which can take longer, you may have to add thinner occaisionaly as you paint.
Andy