that should take you to the bits and accessories area of the Cipher store. The cards part says to specify the card(s) you want...I'm not sure if that means character cards as well. I would presume so, but you may want to mail and ask them about that.
That also links to a bits order area. If the missing head is a normal bit (and not, like, hacked off with a saw) you should be able to get it there.
Sadly, for each figure you have, you didn't get 2 cards...both the character card, and then either an advantage card, or an additional card for the character (usually a skill card or something). You may want to research each character a little bit, and make sure it only had one character associated with it's basic use. What I mean is characters like Cordellia, any of the agents or optional leaders all have skill cards that are needed for their normal use, and aren't optional. I also don't know if ordering one of these character cards will come with both necessary cards, or if they have to be ordered separately. Something else to ask Cipher directly about I suppose.