Marine grade plywood sheets (Weston Underwood DE6)

Hi, I am after a few sheets of marine grade (outdoor) plywood that can be used to cut out to make some outdoor garden sculptures. It doesn’t have to look pretty (weathered or paint-splodged absolutely fine) but needs to be able to remain outdoors (regular plywood or softwood no good). Require sheets bigger than 50cm x 50cm upwards. Circa 15-18mm thick). Said sculptures are intended to be sold in aid of a future charitable fundraiser. Thanks