You can order them on PCBway (I'm not affiliated). I ordered a couple a while back and got it working with the instructions here.

https://www.pcbway.com/project/shareproject/W236257ASZ6_CAM_SDISK2_328.html

Does anyone know how to switch to a different disk image while it's in the SDISK? It has no buttons and now it always loads the image with the latest date stamp.