I use mostly acrylics. The thinner I use is one that MikeV mentioned, and is 2 parts distilled water, 1 part Windex, 1 part Simple Green.
For enamel pick up a gallon of mineral spirits at a hardware store. For laquer use laquer thinner (which also works for enamels or acrylics). Both are about $7 a gallon at most hardware stores. In either of these cases make sure that your airbrush seals and o-rings are resistant to solvents (they should be, but check anyway).
Be aware that you really need to invest in a respirator. There are lots of posts about them here, just search for "Respirator". Breathing paint and thinner vapors is very unhealthy, although I have to admit that I did it for many years.