Agree – make your own.
get a lunch box type plastic box,
put a piece of folded kitchen towel in the bottom,
add a large spoon of water to the towel (you will need to adjust the volume of water added at this stage by trial and error – but do not flood the thing, less is better, you are just trying to build a moisture trap.)
then add a sheet of grease-proof paper that has been cut to fit the bottom of the box.
Add the paint on top of the grease-proof paper and from here collect with the brush.
You should be able to keep a small mix moist for several days, but if there is too much moisture in there, the paint will start to absorb that moisture and run loose, contaminating any other colours you are saving in there.
Keep the lid on when dot in use.