I find cream a bit thick and thin to the consistency of milk. Plain rubbing alcohol works as that is basically what the Tamiya thinner is, but be careful of "Tip Dry" where the paint dries so fast it clogs the airbrush. The brand name thinner has acrylic retarders added, and you can buy the same at art stores like Michael's. It just takes a drop or two per use.
If you do go the alcohol route, I would buy the 90+% version of Isopropyl over the 70% Rubbing, as there is no documentation what the 30% "other" is made up off. It can end up being a substance that can cause the paint to do some pretty wierd stuff.
A good material to test and practice on is plastic milk jugs. Lots of compound curves and it reacts the same way styrene does to paint.