I don't think you can save it because they use totally different formatting structures/file systems- PS1 used the "block" system where each 1mb card was divided into 15 blocks; whereas in the PS2, they use a different file allocation system whereby they measure the storage requirements in kilobytes/megabytes, and each memory card has 8mb of memory.
However, if you just wanted to back data up, IIRC you can use the PS2 browser to copy the files fro PS1 memory cards to PS2 and back again; but your PS1 games will not be able to directly access it.
EDIT: If you are looking for PS1 cards, I'm not sure about first-party ones, but places like Target and KMart which tend to be a little bit on the slow side of bringing in all the new things and getting rid of the old have them. I was at Target in Carlingford Court today and noticed they had PS1 memory cards- I didn't take note of the price as I wasn't interested, but they were 3rd-party memory cards.