There are a few directions you could go in to have the print flex: print from flexible material (although I'm not sure that even the softest grade of Ninjaflex would be flexible enough), or, as someone has already suggested, create a mold for silicone casting. Or, in the worst case, modify one of those sectioned print-in-place flexible snake models, although the result won't be as realistic.

I was hesitating to suggest this, but do you think you could usefully modify a realistic dildo model? Cults3D probably has a few—I think they're the only major repository with a sex toy section.