i happened to stumble upon a great cheap item to use as mixing cups for paint.
searching through the medicine cabinet at work, there were some flexible plastic cups used for irrigating eyes.
http://www.first-aid-product.com/images/M/M795.jpg

they are practically the perfect size to use to fill an airbrush cup with and the flexible plastic and the shape of these cups make it easy to pour not to mention their easy cleanup.
ive been using these for mixing paints, making washes, and using to thin the paints before i pour into my AB