The vacuum diaphragm does not come separately. That is why you can order older valve petcocks without the diaphragm (from older models, but still have the "Reserve" and "OFF" position, which is preferred by many "seasoned" riders) vs new model.
Just replace the assembly, at least you get new valve AND screen with OFF and RESERVE (even PRIME in some cases) if you opt for an older model replacement...for turning OFF the fuel manually for long term storage and less complexity with no worry about leaky rubber diaphragm that goes bad in about 5-7 years. Worst is leaking fuel, letting it drain into the engine and diluting the works, to most common...developing leak so bike runs like it is starved of fuel.
