As an alternative you can still buy small flash drives in bulk (Amazon has listings of 128MB ones for less than a dollar per drive). They don't look as good as SD cards on their own but you could rip out the internals and place them in 3d printed enclosures, which could be an even better result.

I would assume the quality of those things is not great, but the design of this system means they're basically read-only so that should hopefully help them survive longer.