I use parts bins
you get from hardware stores, you can get ones with 40 drawers, even ones with more, and I label them with pieces of watercolor paper with swatches of the color inside. Harbor freight is another place to look at.
You can also get organizers
for nails and hardware, these are good for paint and art supplies, with separate holders which you can take out as needed. They also sell rolling tool boxes
which have removable organizer tops like the above, these could work as an entire plein air setup, or a studio kit. ( These are just ideas, there are a lot of various versions of these at many sizes )
Just go to your local hardware store, or home improvement ware house, and just imagine all the things for tools being used for watercolors instead. The fancy tool bags for electricians and carpenters really work well. Rockler wood working has some nice bags too, and they sell knobs and tracks usable as hardware to make your own easels - T Tracks are really good for those who are good with their hands, but not really good at wood working. They sell tracks which work at 1/4 20tpi which are the same as what you see in tripods.
Also look at items found at military surplus, and fishing/sports stores, often these have lots of compartments, canvas loops and lots of little pockets, especially the items made to store fishing tackle.
A lot of this requires thinking outside the box, I just got a rolling bag with telescoping handle
, the compartments made for batteries fit my mitsubishi pencil sharpener, and there are separate pockets for my paint brushes. It was too much money at the time, but I needed something quickly for a plein air contest the next day, and the current kit I was developing just wasn't going to be ready, and I have basically worn out my previous cart. I just could not see hauling everything I wanted to carry, and this thing carries chairs, food, an umbrella ( the fair is hot, in the sun and all asphalt ), art supplies, paper, water and is big enough for my tripod. In many ways we artists are as much craftsmen and tool enthusiasts as we are painters.
I also got a tool bag
, came with tools, was only $15 on sale, and is perfect for my sketch book, has ample room for lots of stuff, really good as a over the shoulder kit. Pockets are good for storing tubes of paint so they are pretty easy to find, if you palette is limited.