Isopropyl is a suitable alternative thinner for many (notably Tamiya and Gunze) acrylic paints, but it will turn some ( eg. Vallejo) into sticky goo. It can be used to clean your airbrush when using compatible acrylic paints.
Nail varnish remover is acetone (or acetone substitute) based and should clean most acrylics from your airbrush. Not recommended as a thinner though.
Methylated spirits is ethyl alcohol and can be used in the same manner as Isopropyl, but is a little more volatile and evaporates more quickly. I use it both as a thinner and a cleaning medium (I use primarily Tamiya and Gunze acrylics).