Advanced Drainage Systems 4 in. Singlewall x 4 in. x 3 in. Downspout Adapter 0465AA - The Home Depot
Amazon.com : VYH Rain Gutter Downspout Adapter (2" x 3" Inch Gutter to Drain Pipe Adapter) Fits 2" or 3" Inch Outdoor PVC Drain Pipe or Corrugated Landscape Pipe (2" x 3"