Is it 18mm or a non regular size? The MVT is 14mm but the HI is 18mm so if neither of those fit it must be a unique size. Otherwise one of Alan's or Ddave's will fit. The MVT won't fit 18mm.
Mine isn't coming until the end of September and my I heat will probably come then too so I suspect the best stems will be identified by then.
The 18 and 14mm sizes you mention refer to the tapered end of the glass stem, which doesn't fit in the Nomad because the Nomad doesn't have a tapered fitting like Milaana/MVT/etc etc.
Any stem to be used in the Nomad will have to be a straight tube with parallel sides/outside diameter(size currently unknown), unlike the iHeat/MVT stems.
I don't know the OD of Nomad stems, but once we do we can look for tubing or 'straight' stems that will fit.
To use the Nomad with a bubbler we would need a stem with one straight end (size currently unknown) to fit into the Nomad, and a regular tapered end of 18 or 14mm to fit into the bubbler.
(other sillicone or whatever solutions will also be possible , but lets stick to pure glass solutions for the sake of this explanation so as to avoid confusion).
Personally, I will be sticking with the supplied stem anyway as I found the glass screens that I tried on my Millana (420EDC top hats) let a little too many herb particles through compared to regular metal mesh screens.
If an appropriately sized stem with a finer glass screen than the 420EDC top hats becomes available, or is discovered, I would certainly try it though!
And I will deffo try the 420 top hats that I already own, even though I'm pretty certain that they will allow a little too many scooby snacks through for my liking, just like they do in the Mi'.
I hope this clarifies the situation?
Let me know if I've erred.
EDIT:
Loads of posters beat me to it
But I'm gonna post anyway