3.5mm Audio Headphone Jack
I haven't found a distributor, but a (the?) manufacturer is Dalvey Products Supply Ltd.; it's in their catalog here, and the datasheet is here.
It's an unusual component, and it may well have been a production run specifically for Casio. On the other hand, email is cheap and you could ask them if and where you can buy one, or if they will send you a sample.
Other than that, see other answer.