I don't know where the twilight zone is, but I'd take the handle with me to Home Depot. They will likely have the correct nut. If that failed, my local fastener store, pay their usual $5 minimum and walk out with half a dozen. If you're in Ontario, look for Brafasco. If by chance you're in Edmonton, look for Edmonton Nut and Bolt. The nut on the handle is nothing special.
Trying to get RCBS involved just isn't worth the trouble unless you live in a very small community where you don't have any kind of hardware store.