That's going to be a tough find. #40 is an ANSI (U.S.) spec, and the standard sprocket bores are inch, not metric. Easiest way might be to get 20 tooth sprocket with 3/8" bore and drill it out to 11mm, shouldn't be too hard. McMaster-Carr, McMaster-Carr #2737T132, $7.13 for the sprocket in steel. I use McMaster a LOT for all kinds of hardware and materials.