Where would be the cheapest place to buy Plywood?

Your best bet would be to try to find a place were commercial home builders buy products from. It will likely not have a pretty store front and they may not be willing to sell to a consumer with out a commercial account. Their product is cheaper because they usually sell in large volumes and don't maintain an expensive store front. In the US you can usually find them by searching for lumber yards in google. They may call them something different where you live. 200x100 mm Isn't very large though. Is that a typo? In the US plywood usually comes in 4ft x 8ft sheets which is about 1.2m x 2.  more