I would check ebay, you just gotta find the right search words.

I used to work in a used music store, and we had tons of weird jewelry like the kind you're describing. I know for sure we had tons of rings with unscrewable tops to hide stuff in. I'm not sure about necklaces tho.

So maybe just try to find someplace that sells weird jewelry, like hippy shops or used music stores. You never know what you might find there.