I keep all sorts of REAL art supplies on hand, have a dedicated pantry in the kitchen! Here are some tips that have helped me tamp down the chaos of prepping for an art lesson:
I keep my paints in bins organized by color families—primaries (reds, blues, yellows), secondaries (oranges, purples, greens), toasty tones (umbers, browns), blacks and whites.
Drawing materials are all together on a shelf: chalk and oil pastels, charcoal, ink, fixative, rulers, and the oh so vital sundry of magnifying glasses.
I have one shelf for all things watercolor and gouache.
I have an art bin for drawing tools. My children and the students in my co-op are always allowed to dig in. The one rule: Get the tools back in the box!
Every once in a while I do an inventory and I've found that having the one rule works. I rarely find stray pencils or kneaded easers… well, unless the kneaded eraser is cleverly substituted for modeling clay!