The SD card players from Panasonic are now obsolete, you can buy cheap equivalents such as the one here for under £5, or a good quality one with a screen here for around £25.
However in both cases the technology has moved on so they only accept micro SD cards which are 25% of the size, as such you would need to buy a new set of micro sd cards to fit into the device, but could copy and paste the audio files from the old SD to the new one easily.
The last option is to try and find some legacy devices still using the full size SD card such as the one here.
Let me know if I can be of any further assistance.
The links above allow you to buy them on ebay, or if you prefer you can get one here from Amazon.
The micro SD cards can also be pushed from Amazon here for around £7 for 16GB storage.