Try the Parts Shop. Phil quoted me $20 for series designation badges (i.e. E500, 300SL, etc.) He might have the AMG badges as well. If he doesn't have it in stock, I'm sure he can source it.
Otherwise, you might want to look into ebay. I'm not sure whether they are original or not (probably not) but they're going for like $5 or $10 or so. Email the seller to be sure.