e-bay... I bought a box from this guy for less than $20 shipped, decent selection. It was a stack of about 13" x 13" x 3" (one 13x13 piece made my wife a nice cutting board after a trip the the router table.. []
Here's a way to get 'em real cheap .... as in free. Stop by a shop that makes counter tops. Just ask. You'll be surprised at what you can find in their garbage. Don't forget to return to the shop with a couple of pens.