Hmm, that sounds like it might be a bad memory card. Did you try formatting the card in an actual PSX/PS1? (I don't even remember if formatting those old cards was necessary.)

Otherwise, the fact that Sony doesn't sell PSX memory cards anymore is very strange to me. You'd think there would be a demand of some kind. I can't even find third-party products from any large-scale businesses. I have found generic products from businesses like this though:

http://www.handhelditems.com/playstation-memory-card-p-7736.html

http://www.tulipstuff.com/1mb-memory-card-playstaion-ps1-new-yellow-see-through-color-p-9844.html?currency=USD

The trick is finding out if these websites are reliable/safe places to purchase from.