I don't think it is hard to find at all. I want to say the ABCD winner card is the toughie.
I could look at my Thomas cards and tell which I don't have, but I only chased the versions he was on anyway and me not having it doesn't say much anyway.
There are two versions of each though. The redemption card and then the redeemed card that came with the prize. They can be tricky to tell apart from the front because of very small differences that are hard to see unless you are specifically looking, but the back is the dead giveaway.
Looks like I was more involved in the 1994 set. I have the ABCD winner and redeemed (Certified) as well as the single set (can't remember if it was A, B, C or D) and the redeemed (Certified) version. Missing just the double winner (AB or CD) and redeemed version.
For the 93 set, I only have the redeemed D card and nothing else. I don't even have the winner card!