I think that's basically it... It's a lot to do with luck. I spent plenty of time on some other doors as well. Check if you have 99 map cards. If you do, you should go and re-synthesize some rooms. (You probably know this but if you hit a door again I think you can re-synthesize the room.) Then, you will make space for other map cards.
If you have killed all the Heartless in that world, then I'm not quite sure what to tell you. Are you using an emulator on computer for Chain Of Memories because if you are, you can save the state and then reload it, map cards you get at the end of a battle are random and do in fact change. So you can save state just before killing the final Heartless in the room, then keep killing it over and over again until the map card you need.
I'm sorry if I wasn't much help because I can't remember what exact number you are supposed to get. Where is the problem is it that you don't have enough map cards or just can't get the right one?
Well that's all I have to say, maybe someone else can help you. I don't know.
P.S I apologise if you find my post absolutely useless and filled with all the stuff you already know. Thanks for not flaming.