I haven't found anything that looks usable at McMaster Carr, unless
I'm overlooking something. Can a flexible drive shaft be made from a
simple spring, like inside a rotary tool flex shaft? Whatever stuff
used for flexible shaft screwdrivers?

The shaft will be driven by an 18 to 36 V DC motor (up to maybe 700
watts). Not sure the RPM, typical DC motor. Bent 90° in about 3".

Thank-you.