I ordered some from China (Alibaba) a while back that were difficult to work with and obviously did not go through any type of quality control. There was a guy on Instagram out of the UK, William Shakour, I believe he is/was Titan Pens. A couple of years ago, he was selling 3D-printed piston units (picture below) that he uses on his 3D-printed pens. You could contact him on Instagram to see if he is still selling them.
Several other 3D printable piston filler designs are available online that you could try to make yourself if you have access to a 3D printer.
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