I noticed this to upun moving here that finding a birthday card is near on impossible. Once before my Polish was passable I found in tescos some cards and picking one which had a pretty picture of a bird on it and what I thought was Happy Birthday I was happy that I could give this to my wife. Upon reciept however she explained that the card was lovely but the text read "get well soon" ooops one of the many challenges of living here with very little language.
The only market which is strong for cards seems to be cards for christenings communions and weddings as far as I can see
Hope this helps