I make my own by laying up multiple layers of glass cloth on a piece of glass plate. You can use PVA as a parting film, or peel ply. Clamp a board, or something flat on top of your laminate, or vacuum bag it. Put a foam core in the middle if you like, although you could have a balsa core too. Use a good laminating resin. If this is too much work for you, or you don't have all the materials, try to find someone in your area who could lay it up for you.