Consider Husky or Worksman wheels...or custom wheels made by the Wheelmaster. I have Husky wheels with 11ga spokes. Most bicycles have 14ga standard. Most "heavy duty" wheels have 12ga...which are pretty good. That I have seen (there may be others out there) only Husky and Worksman sell wheels with 11ga spokes (Husky sometimes call them "HD-120" because they are .120 inches thick). The Wheelmaster can custom build wheels with 9ga and 10ga spokes. The Husky heavy duty steel rims are strong, but the standard for strength is probably the Worksman.
here are some links:
The Wheel Master
Worksman Cycles - Factory Direct Store: America's Oldest Cycle Manufacturer of Industrial Bicycles and Tricycles, Since 1898
bicycles, tricycles, industrial bicycles, industrial tricycles, bicycle parts, tricycle parts