Any kind of blue or pink styrofoam. It'll be in the building materials section in Lowes or Home Depot. It's available in 2'x8' and 4'x8' sheets. Looks like you'll be fine with a 2'x8'. You can get it in 1" or 2" thickness, which ever is better for you is up to you and your project.
You'll want a tube of GE Silicone 1.
A small woodworking file kit is great for shaping the foam. They had a kit for like $10.99 @ Home Deopt that I bought that contained 6 files rough to smooth, of different styles and shapes. A compass saw is also a great tool, and makes rough cutting your designs WAY easier. Go with cheap tools, afterall its only foam you are working with here.
You'll need a gallon of UGL Drylok, again wither white or gray, it'll be in the paint section.
You'll also need concrete colorant, that'll be somewhere near the cement in the building materials section.
You'll need a 2 cheap brushes as well. After you apply the base coat, wrap the brush up tight in a plastic bag, you can reuse it.
All in all, you're probably looking somewhere near $60 complete.
