I buy small squeeze bottles, (about 1/4 ounce,) on EBAY, maybe $10.00 per hundred. They look much like Vallejo bottles. The top is easy to lift off by pushing it sideways, then lifting it the rest of the way off with a fingernail.
When my paint is thoroughly stirred and mixed, I use a marking pen to place two lines on the bottle side. The top line is for the estimated amount of paint I will need for the job, the lower line is for measuring the amount of thinner for a proper ratio.
Once both ingredients are in place, a finger over the applicator tube gives me a good mix by shaking the bottle, then it's always on hand and I don't have to continually pour and mix thinners for re-loads.
To initially load the bottle, I use anything like a toothpick to hold against the original paint jar side, with the toothpick ending in the applicator bottle. The poured paint will then neatly follow the toothpick directly into the second bottle, in a narrow stream conforming to the thickness of the toothpick.
From experience I can say, brace the bottle you're pouring into for holding it steady. I use other bottles to surround the one I'm pouring into, to keep it in place. Being so small and light, it's easy to make a mess by letting it get moved out of position.
Works well for me, good time saver, it'[s worth a try.