Art Boards Round Wood Archival Art Panels are made in 10 stock size diameters and any custom size.
500mm Round Art board / Panel ! FixMaster
Gwartzman's Professional Wood Panel - 7/8" Reg. - Round - 20" Diameter – Gwartzman's Art Supplies
USA Custom Sized Cradled Birch Wood Panels/Boards: Rectangular,Round,Hexagon,Ellipse,Triangle,Pentagon,Heart,Star,Star and Ring Shapes
Small Cradled Canvases, Set of 3 Nesting Trays, Unfinished Circle Wood Panels for Painting, Wood Signs, & Resin Art, by Woodpeckers : Amazon.co.uk: Home & Kitchen