I'm not sure what you mean when you say "by 20". If you mean a 20 mm wide rim I can't help you but why would you want so narrow a rim with the load you describe?
40 hole rims are primarily the venue of tandems nowdays. Peter White lists several rims that you might consider.
http://peterwhitecycles.com/
These would include the Sun CR18 which can accomodate tires as narrow as 25. Also see the Velocity Dyad and the Mavic A719 which can take 28 or wider tires. I wouls also think your LBS could order them for you. Hope this helps. Len Diamond Ridgewood, New Jersey USA