Reddit mentions of Astrobrights Colored Paper, 8.5" x 11", 24lb/89 gsm,"Vintage" 5-Color Assortment, 500 Sheets (21224)

Assuming Kindergarten means five year olds, you'll want lots of basic arts and crafts supplies - crayons, markers, pencils, glue, safety scissors, maybe even paint. Washable is a must at this age.

Colorful copy paper is multicolored paper that can be put through a copy machine - something like this: http://www.amazon.com/Neenah-Astrobrights-Colored-Assortment-21224/dp/B00006IDQR. Construction paper will rip or get clogged in a copy machine; it's too thick.

Kids at this age are just probably getting introduced to letters, reading, and early math concepts. It's an age for simple abstract concepts - shapes, colors, opposites, etc.

There's actually a big range of books you can get away with at this age. Kids who are being read to are old enough to sit through a long-ish book (Bernstein Bears, Amelia Bedelia, even Henry and Mudge). At the same time, they're only just learning to read so super easy books are good for reading on their own.

For books specifically, I can hook you up with used kids books at $1/ea. I think shipping is relatively affordable via USPS media mail. I can't guarantee specific series/authors, though I'll try.