Im not sure about your hookah in particular, but that design and style are authentic Mya. It is relatively knew, to my knowledge, and few people carry them yet. They are beautiful IMO.
Your best bet for a downstem is calling Mya direct, most retailers stock a few replacement parts but they are usually for people who order from them and receive damaged or incomplete products so you may be hard pressed to get someone to ship it out to you.
On a side note, $20 is dirt cheap for that hookah. That price is below most wholesale prices so you got a great deal, but it also makes me wonder how authentic it is.
Hope this helps.