Your dealer should be able to help, should just be a change of sprocket to a 3/8 x 8 tooth. This is a similar size to the .404 x 7 and will keep the gearing about the same. I'm surprised you can't find anyone selling made up ripping chains online, there is a forum called atbtalk, be worth asking on there, its an open forum so easy to join.
Had a look at the Arbtalk forum brilliant forum if you like wood.
TC
yeah ive had a look and joined, there some very knowlegable people on there and some lovely examples of some british hardwoods! some of the elm slabs that are on there had me