1$ per 12"x12" sheet not a bad price
http://ocoochhardwoods.com/plywood.php
Great price, shipping on 30pcs is only $13
1/8" X 12" X 12" Baltic Birch Plywood Great for Laser, Cnc, and Scroll Saw. 45pc Woodpeckers® https://www.amazon.com/dp/B013NT3OAC/ref=cm_sw_r_other_apa_EOe2xbZP64X1P
That is about 10c cheaper per sheet but you can’t pick the quantity you want
Since the stock K40 does 8x12, I buy $2 12"x24" from Ocooch, then cut them down to 3 pieces of 12x8. $0.67 each. I tried using 12x12 pieces before, and having to flip them to use the remaining 4" just wasn’t worth it. A lot of times I don’t have pieces small enough to fit on that, nor can it cut something larger than 8" in one axis, so why bother. My opinion of course. YMMV.