The Wiki has a lot of lovely scans, most I think provided by Breyer600.
http://mylittlewiki.org/wiki/Main_Page. Just look under the individual ponies and scroll down to "backcard".
Is there any one in particular you are looking for? I have at least one card from most sets either in person, as scans or even as photos [I didnt have access to a scanner for a while unfortunately] if it helps at all.