They do have empty cartridges that are designed to be filled...
https://www.amazon.com/Empty-Glass-DU215-Holes-Design/dp/B07XW5M2T4/ref=sr_1_10?keywords=empty+cartridge&qid=1569105454&sr=8-10
https://www.amazon.com/King-Style-20pack-Empty-Cartridges/dp/B07XDFJH12/ref=sr_1_13?keywords=empty+cartridge&qid=1569105510&sr=8-13