I think you're SOL. The only place I even found that sells them without the entire lock rung assembly was Duosafety, see 1-scrollbar click down on their Miscellaneous Parts page (
) at $5 each. Not horrible, but would likely need to be doctored to fit your ladder.
As an alternative or even first try. I'd probably get the cheapest Spring Clamps and tear them apart. Here's a Home Depot example
worth a shot at $1 a pair and with some doctoring may work just fine. Either option will need you to play with the stretching out to get the right tension.