Home Depot has latex tubing, though they may not have the right size. I’m attaching tubing to the plastic fittings of a regular melodica tube. If it goes into the end of the melodica, the tubing will have to fit tightly, and even then it may pop out when you’re playing. But you are thinking of using tubing to extend the adjustable tube. You’re talking about the strange looking yellow tube-mouthpiece, right? I haven’t seen one in person, but it looks like the end of a latex tube of the right inner diameter could slip over that. You might take the adjustable mouthpiece to Home Depot or a hardware store and just try the ends of whatever rolls of latex tubing they have. The alternative, which seems simpler under playing conditions, is to have a separate long Hohner tube.