Euclid probably makes the spindle nut you are looking for. Napa has access to them, but most counter people don't know how to identify them. The one you need is probably on this rack. https://www.ryderfleetproducts.com/euclid-e7648/display-board-only-spindle-nut-p-439-e7648

If you are near a trailer dealer, they probably have access to Euclid. If you are not near a trailer dealer, measure the spindle size and the threads per inch and call one of these guys locations. https://www.ogburns.com/contact.cfm Most of their locations have pretty competent people.