Not all of them are labelled as tentacles, some are called tails or monster hands, but they’ll all do the job. There are quite a few tentacle rigs on thingiverse you could simply print and assemble to play with. You can get more complicated with it, but simple works as well. There’s a stack of vertibrae that pivot along a center line and cables running along the length to pull and supply tension.
While a tentacle has what could be perceived as a quite complex motion, the construction of an animatronic one is actually fairly simple.
