Try Taylors of midfield or Beaufort ink.
By the time you have paid for postage of a set of tubes, you may as well scrap the kit though, depending what you paid for it. Taylors do some nice kits, so you could buy some other bits whilst getting the tubes- helps spread the postage if nothing else. I am also not sure if all Jr gent tubes are exactly the same size- I have a feeling that there is a new and old version.