Try doing a search on model power boats. Some of them use flex
shafts, and the larger ones might be usable.

You could also search on flexible shafts. There is at least one US
company that makes them (S.S. White???), and their website will help
you select what you need.

Howsomeever, it sounds like you want to transmit a LOT of torque over
a tight radius. Consider bevel gears insteads.