Recently I had the bright idea to use squeezable plastic dropper bottles for premixed (Tamiya acrylic) paint. Here in Korea, they're a staple in every pharmacy. There's no such thing as "2 teaspoons" for liquid meds. They tell you precisely how many CC's you're supposed to take, and they send you home with a little plastic squeeze bottle with a built in measuring chart.
They're dirt cheap as pharmacies buy them in the tens of thousands here, but they were a little tricky to get since they are a resource and not a saleable product for the pharmacy. After a few sketchy trips to various local pharmacies, leaving with a small handful of bottles for a $1 or so and some puzzled and slightly suspicious looks, I hit upon a source. A friend of a friend who works at the pharmacy got me a bag of 100 for $10. They work like a charm too.
Since they're used for meds, the plastic is very stable and can be used for several products. Also, the 20ml size and the built in chart make mixing ratios a breeze - 5ml of paint... add enough thinner to top it at 7.5ml.. done!
Also, the fact that they can dole out a single drop at a time, make them perfect for loading an airbrush efficiently. Oh, and I added a couple of glass beads to each and shake up mixing is easy peasy.
Heh, and the best part is that because the little pink caps are plastic, they're easy to spray with the contents of the bottle... so you have an on-plastic colour reference at hand, and it makes spotting the ones you want super easy too!
A quick online search for squeezable plastic dropper bottles will bring up several options. If I run across a good online source I'll post it up.