Office Max is where I find the paint and the paper. It's a little larger and more durable than the usual stuff you find in the stock room at your school. I also invest in acrylic paints. I then usually enhance these basic colors with a couple different shades of brown and purple. I really like to affect acrylic paints have over the tempera of poster type of paints.
Students can get much more texture and vibrant color with acrylic.
I also keep on hand a couple of packs of basic water colors which come in handy for backgrounds and softer uses of color. This past year I implemented a new rule that during the last 15 minutes of class students were to check to see if anyone had any extra paint before getting more. This worked out really well as kids shared the excess paint, thus not wasting any and making clean up easier. I have gathered throughout the years. I also drain the shelves of Art books at my public libraries.