Hi snoids the cards you are on about are the voice data cards and the waveform cards. They act like the nintendo 64 games catridges. Just plug them in and choose utility card and load to download the sounds.
Now where to get them, to be frank they are like (rocking horse shit). I managed to get some shorttly after I bought my sy77. I got them in a place in Cardiff called MusicLand.
I hope this answers your Query.