I always thought it was called a tie bar (as opposed to a tie clasp, or tie clip, for holding the tie to your shirt further down).
I used to wear one a very long, long time ago.
I'd suggest contacting a good jeweler or men's haberdasher. If you really got to have one, you'll be able to get one custom made for sure. It's only three small parts, two of them silver soldered together, two of them threaded. If you find a good precious metal smith, you'll probably be able to design your own and he'll craft it for you. I doubt that it will cost very much.