Search for 'Embroidery Memory Card Janome 9000'
Ebay is the most likely place to find some. Not many available in the UK, more available in USA and then Australia.
Two memory cards came as standard with the machine. The file format is .SEW and whilst there was an extensive range of memory cards created, they were always quite expensive.
My 9000 is still going strong although and I still love using it - beautiful stitch quality.
About rewritable cards and kits-
Several Years ago I did look into finding a read / write card kit but there were none other than the ones that were used with the Customiser 2000 software . Finding a blank re/writable card to use was like searching for hens teeth.
Eventually, I acquired the right kit and set about working with the legacy software with Windows 7. A key issue was how to slow down the transfer rate of a modern computer to match the maximum the card reader/writer could handle. Another was to keep an old computer going to run the software. (When I got a different machine for embroidery, I passed on the read/write card kit to another 9000 owner.)
I hope that you manage to find a set of compatible kit, but do be aware that it probably won't work with a modern computer.