They only replace the lozenge ones as far as I know, and only have the metal ones, silver or gold. They will send them to you for free if you ask. Most boutiques seem to have a collection of leather hangtags they give out when asked as well. Since the metal carriage tag is a more specialized thing, you'll probably have to buy one off Ebay.
I bought a lavender Maggie last year from my local outlet, which was missing its carriage hang tag (unnoticed by me when I bought it) but when I realized it and went back, they didn't have any spares, nor did my local FP boutique....At the time, the outlet manager mentioned they were really popular and tended to be "removed" (read:stolen) by sticky fingered customers I ALMOST resorted to buying one on eBay, but they go for a ridiculous amount of money, so I'm fortunate enough that I have another Maggie w/ the same H/W color (silver), so I swap them back and forth as needed....it's kind of a pain, but oh well.
The factory stores are usually the best bet. I have asked Coach on the phone and at Coach stores before and they never have any of the unusual ones. But when I carried my purse into a factory store, the SA said "let me get you a hangtag for that" and she did. I bought my Poppy hangtags on Ebay for about $8. It actually was a great deal because I bought the Poppy on Ebay too and it sold very cheap because it didn't have hangtags. I never tried to get them at a factory store since I live so far away.