Hi Joel.

The part you are referring to is called the Nut.

A nut is made by filing a blank to custom fit a specific guitar; you don’t just buy them off the shelf. You just need to find a luthier locally and let him install a new nut. Ask at your local music store. Most music stores these days act as pickup/drop off sites where a luthier collects and returns guitars he works on. Some stores have a repairman on site, but this arrangement seems to be dwindling.

It’s been awhile since I’ve had this done, but I’m thinking it is a $25-$35 repair.