In the past, it's usually been easiest to catch Mary at her desk between 10-1 Pacific. Not on Fridays when she's packing orders.
Here's my past experience ordering a replacement nut... (I wanted to have an alternate one for my Distillate for B-E-A-D tuning...) They can sell you a brass nut blank, but each one has to be custom-fit to your bass. No two of them are identical. If you are handy at all, it isn't hard to do. Any well-qualified repair shop should be able to shape and fit a brass nut for you if you aren't comfortable doing it.
*back up just a little... is yours missing completely or just somehow not working?