Used Styrofoam billets. 10" x 20" for the most part. Good for floats, floatation for boat rehabs, etc. I have a few more that are not in the picture. New 8 footers at a local lumber yard are around $333. These ones are $10/foot, cash. Less if you buy it all.
I currently have on site:
1 8' piece
1 6'
6 5'
2 4'
6 3'
Assorted smaller pieces