barsto.com
Barsto will make them for you with options on thread pitch, crown type, and fitting preference (drop-in/fitted) they run 315$ a barrel and are made to order. I do not believe that anyone else (including sig) makes a 40 P229 threaded barrel. P226 barrels are not interchangeable with the P229's (think a Glock 17 threaded barrel in a 19) but I have heard they will "run" without 100% lock-up.... but that just sounds like a bad idea (just a heads up in case barsto no longer offers the service)