Amazon.com: Classic Dark Grey Baseplate, 1 Piece 32x32 Building Base Plates-10x10 in, Compatible with Lego Parts and Pieces 11023(1 Pieces,Dark Grey) : Toys & Games
Amazon.com: LEGO Classic Gray Baseplate Square 48x48 Stud Foundation to Build, Play, and Display Brick Creations, Great for City Streets, Castle, and Mountain Scenes, 11024 : Toys & Games
LEGO part 51542 - Light Bluish Gray Baseplate, Raised 32 x 48 x 6 with Level Front at BrickScout