Hello, AJ, and welcome. We have had some discussion about mouthpieces and tubes, and I have done some research on the topic. The Melodyhorn, like most regular melodicas, comes with (1) a mouthpiece that’s used independently of a tube, and (2) a blowtube with fittings at each end, a mouthpiece fitting and a fitting that inserts (as does the regular mouthpiece) into the melodica’s blowhole. (I don’t want to belabor the obvious, but do want to be sure I’m clear.) The blowhole diameter is not uniform. A Yamaha mouthpiece or tube blowhole fitting, for example, won’t fit into a Suzuki melodica. Those fitting sizes are shared by some melodica brands but not by others. The Suzuki size is more common. The tubes themselves, though, are interchangeable, so that fittings from any (almost any?) melodica will fit into thye ends of any (almost any?) tube.
Suzuki and Hohner melodica mouthpieces and blowhole fittings are interchangeable; though the fit isn’t exact, they work well enough. The Melodyhorn mouthpice looks like a Hohner mouthpiece, and my best guess would be that Hohner mouthpiece and tube would work. melodicas.com sells Hohner accessories. Amazon (USA) sells Hohner mouthpieces for about $8. The black tubular one, not the stubby cream colored one.
The Angel Melodyhorn part number for their mouthpiece in MP008002, but I can only find it on a foreign site (Greek, I think).
Rakuten (Global) from Japan also sells some mouthpieces and tubes. They’re good, but delivery is too expensive for such stuff.
I personally hardly ever use the tubes of my melodicas, and the more I play the less often I even use a mouthpiece. I started out playing without either so I’d learn to play without looking at the keyboard. It worked, and I’ve gotten so I’m comfortable blowing directly into the melodica, which I also think produces a better sound. A curved mouthpiece allows you to sort of glance at the keyboard.
Anybody else feel free to correct me or provide better information!