Like I said earlier, I once used a pot of interior latex specifically for its physical properties.... it didn't cover well and it did go on very smoothly. This let me layer-up white; the miniatures paints I had at the time were chalky and covered unevenly, as well as making physically uneven layers.
I have looked at interior latex paints by the conventional 1-2 litre pot, but they cost too much per single colour and don't come by mail order...