Ya finding a vintage one will be hard. I'd love to find a Forest Subaru one for my car (the dealer in my hometown when I was a kid)
Something odd to try, but it just might work: Look at craigslist ads in Kennewick, Pasco, Richland WA (Tri Cities) If the pictures are decent MAYBE you can spot an old car for sale with the plate frames. Contact the seller and offer to buy the frames for $20 or something. It would be a super long shot though.
If you don't mind it being new, then as said above, just contact the dealer.