The first thing that pops into my mind when I see a dealer nameplate on the back of a vintage Pontiac is, "Wow! It's a low-mileage original that probably lead a pampered life and has all sorts of other cool paperwork to go with it." Why? Because most dealer nameplates wore out, broke off, were removed and trashed by the original owner, or were never reinstalled after a restoration. To see one still on the vehicle or returned to it after a resto is a subtle clue to a charmed life.
If your plate is missing and you'd like to get another one, swap-meet areas at car shows, club classifieds, and online auctions regularly offer them for sale. If you know the selling dealer of your Pontiac, you can search those places on the chance that one turns up. It's a long shot, but it may be fun.
Instead, I would begin with an Internet search to see if the selling dealer is still in business. If so, call and ask if it regularly installed nameplates or name decals in the past. If it didn't, your search is already over. If it did, ask if it still does and whether you can get a new one. Keep in mind the new one probably has a different design than the one that would be correct for your vintage Pontiac, as many dealers changed from metal to decals and updated the look over the years. You may be surprised, however, to find in rare cases it's the same.
If it's not, try to play on the emotions of the dealership personnel. Seek out an employee who is into old cars by either going there or asking over the phone. Tell him about your Pontiac. Then ask if there are any old dealer nameplates like the one you need, hanging around the dealership. Chances are, if the dealer has not changed names over the years, some longtime employee will have one in a desk drawer or up on the wall just waiting to be purchased and installed in its rightful place, your vintage Pontiac.