I had seen some of the old Pokemon parts for the DS on eBay and there were a lot of repro versions, i.e. Imitation cartridges on which the game was loaded.
I wondered if they could be bought somewhere. I can't imagine now that every single one of them makes these things themselves.
Does anyone know where to buy them?
Of course you can buy something like that.
When you buy a repro card, in most cases you can write to this cartridge directly. (Of course that depends on the storage type).
So-called flash cards, which have a slot for a micro SD card, are particularly user-friendly.
You can usually find used flash cards from 10 euro.
I have a flashcard myself (R4 Gold Pro), but I wanted the individual Roms on a cartridge that starts the game directly. I also have the problem with my flashcard that every now and then all games are deleted. I just don't know so I can get some.