Hi Dos Tazas,
Alan has really fixed you up here with good information. I have been experimenting with tubing and mouthpieces for my Piano 26/27/32s and found 90 degree copper elbows at the hardware store that fit the the hole in these instruments. Then all you need to do is find tubing that fits the elbow (and perhaps a mouthpiece that fits the tubing if in fact you want a mouthpiece). I have posted a number of photos and recordings concerning these Piano 26/27/32s and they are my favorite melodica. I really don’t play anything else.
These instruments are pretty easy and responsive to play without any mouthpiece. You just press you lips up against the hole.
Unless you want to play using my technique or with two hands, I would not recommend using a tube. The rigid mouthpiece that is usually shipped with the melodica works pretty good.
A tube of the right diameter and of the right thickness/flexibility can be tricky to find.