"Unnecessary?"
I'd call them a diving impediment.
A reef rod, or a simple stainless steel or aluminum rod can be handy for use when steadying a camera in a narrow set of circumstances but otherwise, just another gimmick scuba accessory that has little to no value to the regular recreational diver.
If you must have one, any hardware store that sells aluminum or stainless steel will have them. Cut to length and attach a wrist strap, use until you realize you dont really need it anymore, then throw it on the ever growing pile of things you bought that you never really needed for diving.