As you are finding out, these are totally obsolete and have not been made for years. Sorry.
You want a PCMCIA Type I SRAM card. Any kind of flash, dram or ATA card is not suitable. No point in getting anything bigger than 4MB.
I had a look on eBay, and there are a few items described as "FUJI SRAM Memory Cards."
These are worth looking at, but I cannot confirm if they are any good.
Or try an eBay search for "pcmcia sram" – this will give a lot of options.
Replace the battery as soon as you get one (unless you are lucky enough to find a rechargeable).