What kind of antenna are we talkin about? A screw in one original to the phone? If so which phone model
i would start with Samsung, but Amazon, eBay or the cellphonestoreonline.com may have what you need. The model number is important.
>Can I buy a replacement?
Sure you can. There are many 3rd party vendors that sell antennas besides VZW. For example www.3gstore.com