Johnny on the spot
Someone ready and available at the time needed.
The term appears in an article the New York Sun, April 1896 - "JOHNNY ON THE SPOT A New Phrase Which Has Become Popular in New York". This was widely syndicated in the US at the time and there are many citations in US newspapers from shortly afterwards.
The OED has this citation, from George Ade's, 'Artie', also from 1896:
"A Johnny-on-the-spot... was tryin' to keep cases on her."
The name Johnny doesn't here refer to any particular person - it is just being used as a generic male name. So also, Jimmy Riddle.