The best replacement for flex shaft is stiff shaft.

The best alternatives to flex shaft are rated as following:
★★★★★  stiff shaft
★★★  regular flex
★★★  dremel
★★  stiff
★  stiff flex
★  senior flex
★  regular
  stiff flex shaft
  regular flex shaft

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Speaking about flex shaft, one probably shouldn't forget about dremel tool.

Other search terms related to flex shaft and their ratings are:
★★★★★ dremel tool
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flex shaft
stiff shaft
regular flex