Favorite sources for cheap books include the brick& mortar Half Price Books clearance section (and did you know homeschoolers can receive 10% off?); Amazon, eBay, and local curriculum sales.
SCIENCE!Magic School Bus DVD box set. All 52 episodes! There are many that will tie in right with our My Father's World kindergarten units, so that's cool. I grabbed it on Amazon when it was $39.
Awesome Ocean Science (ages 7-12)
Backyard Scientist series one: 25 experiments that kids can perform using things found around the house (ages 4-12)
Big Book of Play and Find Out Science Projects (ages 4-7): topics include physical science, nature, bugs, human body (Timberdoodle PreK core)
Flip-Flap Body Book (Sonlight prek)
My First Super Science Kit - activities for ages 4+
Science Play: Beginning discoveries for 2-to-6-year-olds
The Usborne Book of Science Activities vol. 1: water, magnets, light & mirrors. (Sonlight grade 1 and Science B)