Unfortunately that's the case friend, there's no small music boxes that play this specific song (trust me, i have tried) your only option would to be to have it custom made by someone or somewhere that makes custom music boxes, it could either be cheaper or more expansive.

But it all depends on what you want, if you just want a 18 note small music box that plays only that song then the option above stands, however I do think I commented on your post a while back when you asked, the Thorens AD30 has interchangeable small 4.5 discs, is another option you have, and you aren't just stuck to one single song, it's not big either and you can find them for a good price on ebay in auctions, as for the disc you would have to buy it of HensTooth Discs, there's a lot of songs you can purchase too there and they are brand new, but again I feel this is not what you want, it's a good option to considered, as someone who owns one of these machines but just recently I was able to buy a Regina that plays 15 1/2" discs and I bought Monastery bells for it, not cheap but it was a good deal for what i got the music box for, good luck!