I cannot help you directly, but you may want to check the fan sights and see if a plant has centaur treasure cards as a possible harvest. I know I have harvested minotaur and earthquake from gardening in the past. A fan site may also list boss and regular monster drops. Perhaps there is a boss or a monster that drops a centaur card. Those are the only other possible suggestions I can think of.
You know, someone could just flat out lie to you. If you don't research the fan sites to make sure, you might not know that it was a lie. All they have to do is say, "I think I got a centaur card from -boss name- in celesta/winter tusk." "I think I remember getting that card from my -plant name- at ancient." It could be that the player might not be malicious, they could just be mistaken. In which case the moderators are rather unlikely to fly in and say, "You are mistaken young wizard! You cannot get centaur cards from there." From what I have seen from the pet forums the mods let all kinds of wild speculation take place, never saying if any one person's post is true or false. Someone could just pop in and say, "you can't get centaur cards anywhere unless you know a life wizard, just give up, I have searched all over." This is the internet you can't trust any answer, some people are just lie and enjoy causing frustration.
Why did I post this wall of text? Because, it seemed to me, that your replies to the other posts may be thought of as ungrateful, by people who actually do know where centaur cards are. If people think that you are going to be ungrateful, they may decide not to answer. They may decide, form your previous replies, that you are mean and don't deserve the answer handed to you. Sorry if you don't like to be corrected, and if it sounds like I judged you solely from reading two sentences. (I said I am sorry, so you can't get mad at me for saying the things I said right? I mean I could kill the last Galapagos Tortoise, but as long as I said I was sorry afterword, one would be allowed to be mad at me ... right?)
Rarely from drops or bazaar, check on central or wizard101 wiki to figure out where they are dropped. Otherwise, if you're life school you can craft them. If not you need a life friend to craft them for you.