Been having the same problem here, went to like 8 stores, only 3 of them had the code cards advertised, but not a single one had any in stock. a lady at Big W in sunbury did give me 2 code cards for the shiny gengar though which surprisingly still worked.
guess I'll be missing out on Diancie, oh well.
All things considered, Generation 3 was good, but it came out just as my interest was starting to wane. so it ranks lower for me. but from what I've played of Omega Ruby so far it is REALLY REALLY good, that new Dex Nav feature is really nice. (search for pokemon in the wild with rare moves and abilities, etc) I haven't made it very far yet but I'm loving the remakes a LOT more than the originals.
I don't know for sure who are version exlucisves besides the cover legendaries though.
and I went with the Water starter this time