Welcome to the site, Cashew.
Please read back a few pages in this thread, we discuss the variety of options that the HA allows for vape bonging. Some use the solo stems, some get the adaptor from planet vape, or direct from HA. Some peeps prefer to use 18.8 mm bubblers or bongs (same as 18 or 19), some prefer to use 14.4 mm (same as 14).
I prefer to use the stock mouthpiece myself, which fits inside my 18.8 mm bubbler, but this method isn't as stable as the adaptors since the mouthpiece simply snaps into the HA itself, and can come out, if much horizontal force is applied.
It's all about personal preference and which method you like best.