go to home depot or an autoparts store show them the bolt and say i need this.....if that doesn't work, go to the dealership. if i remember correctly that a beefy bolt, so i don't know if a home improvement store would carry it. i would start with the autoparts stores.
edit, just checked courtesy parts for ya, they don't list just the bolt for th rears, only the front seats, not sure if it's the same, but that may be something to check
Wow, that's gonna' be a touchy one.. Seatbelts, and their workings, come with a lifetime warranty,out of safety concern. Being that you took a bolt out/off for something, I wonder what the reaction will be in regards to replacement.. Free, or no free?