Beckett is the website for finding out card values... costs though, otherwise as suggested best bet is to search recent sales on ebay.
Also this website will just sell them for you:
http://www.comc.com/
it's pretty good but best for cheaper cards you just want to get anything for, they won't really maximize value and obviously they charge for the service. I believe you just mail the cards to them and they will consult beckett and price them accordingly.