OK, when my dad was having trouble getting into and out of the tub, we looked for grab bars at the places listed under "Medical Supplies" in the yellow pages. Those places would also know of all the stores that sell stuff for "Assissted Living" as well.
I recall seeing grab bars that clamped onto the outside of an enameled steel bathtub. I see no reason why you couldn't put a 4X4 block of wood under that clamp to clamp it onto the side of a stand alone cast iron bath tub.
I'd take a look at grab bars of this design:
and see if you can't put a block of wood on the outside of the tub to attach it securely.