You can get DX code stickers from Porters Cameras, they sell them in various speeds, if you have a local lab that processes E-6 film, it is pretty easy to find ISO 50 DX encoded empties from fuji velivia film, I have hundreds if not thousands of them in a box here.

The reason you find what seem to be re-used cartridges in the single use cameras is kodak used to have a cartridge re-cycle program, we used to save all of our cartridges and send them to kodak and they paid us like 3 cents each for them, then they were reloaded and put into the single use cameras, there are also many companies around that recycle the single use camera bodies and reload them and sell them under store brands.